Sources of information

Materials in inner layers are generally denser. Exam tip: connect density with layer structure.

The direct answer is A: oceanic crust. Rocks from the ocean floor are physical material from, or closely associated with, the crust beneath oceans. Scientists study their minerals, chemical composition, age, magnetic patterns and arrangement. Step by step: a sample is collected from the seabed, its rock type and minerals are identified, its age and magnetic record may be examined, and these observations help explain the nature and history of oceanic crust, including processes such as seafloor spreading. Option A is correct because ocean-floor rocks directly provide evidence about oceanic crust. Option B is wrong because atmospheric humidity is measured with atmospheric instruments, not inferred as the main purpose from seabed rocks. Option C is wrong because agricultural production concerns soil, crops, water and farming practices. Option D is wrong because the number of towns is a settlement or census fact. The rocks may also help study plate movement, but the asked source specifically informs us about oceanic crust. Memory cue: ocean floor rocks belong to the oceanic crust.

Volcanic bombs and ash give clues linked with internal materials. Exam tip: keep them under volcanic materials.

Seismology explains internal structure from wave behavior. Exam tip: connect seismology with seismic sources.

Artificial waves give controlled clues about underground structure. Exam tip: treat it as an indirect method.

Present mining and drilling do not reach the core. Exam tip: consider core information mostly indirect.

Seismic waves indicate state and magnetic field indicates metallic nature. Exam tip: remember combined evidence for the core.

The direct answer is option A: ask whether a source gives a real sample or only clues. A direct source provides material that can actually be examined from Earth’s interior, such as rocks obtained from mines, drilling, or volcanic eruptions. An indirect source does not bring the interior itself to the surface; instead, it gives evidence from which scientists infer hidden conditions. Examples include seismic waves, gravity, magnetism, and heat flow. The reasoning is: first ask what the source supplies; if it supplies actual interior material, classify it as direct; if it supplies measurements or effects that require interpretation, classify it as indirect. Option A is correct because it gives exactly this test. Option B is wrong because beauty has no scientific role in classifying evidence. Option C is wrong because a source’s location in a school says nothing about whether its evidence is direct or indirect. Option D is wrong because cost is unrelated to the nature of the evidence. A specimen can be inexpensive and direct, while an expensive instrument may provide indirect evidence. Memory cue: sample means direct; signal or clue means indirect.

Seismic gravity and magnetic studies are clue based sources. Exam tip: keep them in the indirect group.

Mining drilling and volcanic material provide real samples or materials. Exam tip: keep them under direct sources.

The direct answer is A, a new layer or a more rigid medium. P waves are compressional seismic waves, and they can travel through solids, liquids, and gases. Their speed depends on the material’s density and elastic or rigid properties. If P-wave speed suddenly increases, the wave may have entered a different layer whose material transmits the wave faster, often because it is more rigid or has different physical properties. Option A is correct because a sudden speed change is useful evidence of an internal boundary or a change in material. Option B, rainfall stopping, is a surface weather event and does not normally cause a sudden internal seismic-speed increase. Option C, a river drying, concerns surface water and is unrelated to deep wave transmission. Option D, crop harvesting, is an agricultural activity and has no role in P-wave speed. The exact interpretation can depend on density, temperature, pressure, and composition, so “may indicate” is appropriately cautious. Memory cue: a sudden seismic-speed change is a clue to a layer boundary or changed material.

Reflection is the return of a wave when it meets a boundary or surface and does not continue into the next medium in the same way. The returning wave may be light, sound, water, or a seismic wave. The boundary changes the wave’s direction back toward the original medium. This basic idea is used in mirrors, echoes, and the study of earthquake waves inside the Earth.

The question asks specifically about a wave that returns from a boundary. That definition directly matches reflection, so choice B is correct. Refraction means the bending or change in speed of a wave as it enters another medium; it is not simply the return of the wave. Condensation is a change of gas into liquid, and deposition is the settling or accumulation of material. Therefore neither of those choices describes the wave process.

Refraction is the change in direction or speed of a wave when it enters a different medium. A medium means the material through which the wave travels, such as rock, water, air, or another layer inside Earth. If the wave speed changes at a boundary, its path may bend. This bending is called refraction. Seismic waves often show this behaviour as they pass from one layer of Earth into another layer with different properties.

Reflection is different: it is the return of a wave from a boundary rather than its bending through a new medium. Evaporation is the change of liquid into vapour, and weathering is the breakdown of rocks at or near the surface. These meanings do not fit the question. Therefore option B, refraction, is correct.

Wave ability in media helps infer solid and liquid states. Exam tip: remember S waves do not pass through liquids.

The direct answer is A: mines do not reach Earth’s centre. A mine is a human-made opening that reaches only a small depth compared with Earth’s radius. Therefore, rocks obtained from mines tell us about only the shallow crust, not the deep mantle or core. Scientists also use indirect evidence such as earthquake waves, gravity, magnetism, volcanic materials and meteorites to understand the interior. Option A is correct because mines are too shallow. Option B is wrong because mines do show and provide rock samples. Option C is wrong because mines are certainly found on Earth. Option D is wrong because mines do not form clouds; this is unrelated. Remember: mines give limited direct evidence, while deep interior is studied mainly indirectly.

Scientists combine direct and indirect evidence to draw conclusions. Exam tip: remember the importance of combined evidence.

Meteorites indirectly help understand composition of Earth-like materials. Exam tip: do not treat them as direct samples.

Gravity anomaly may indicate density differences inside. Exam tip: remember it as an indirect source.

Magnetic properties are linked with metallic materials. Exam tip: use this to understand nature of the core.

Rock analysis gives information about minerals and elements. Exam tip: remember rock samples for chemical information.

Temperature and pressure show internal physical conditions of Earth. Exam tip: connect them with structural study.

Lava gives internal clues but is not a direct sample of the core. Exam tip: also write the limitation of the source.

Conclusions become more correct by combining many sources. Exam tip: treat combined study as scientific approach.

A source and its evidence must be related to the information being studied. Drilling can provide a rock sample that is directly observed, so it is a direct form of evidence. Seismic waves travel through Earth and change speed or direction in different layers, making them an indirect source of information about the interior. Gravity measurements can also reveal differences in density because dense materials affect gravitational values differently.

A rain gauge measures the amount of rainfall. It is useful for studying weather and climate, but it cannot provide evidence about whether Earth’s core is liquid or solid. The other three pairs show a reasonable connection between a method and the evidence it supplies. Hence option D is the incorrect match.

Scientific study first understands sources and evidence. Exam tip: keep conclusions based on evidence.