Sources of information

A mine provides real rock material so it is a direct source. Exam tip: link real sample with direct evidence.

Seismic waves pass through Earth and give clues. Exam tip: write clue based source as indirect.

Drilling gives real samples and temperature data but depth remains limited. Exam tip: remember both use and limitation of drilling.

Volcanoes can sometimes bring deep rock fragments to the surface. Exam tip: treat volcanic material as useful but limited source.

P wave velocity changes with properties of the medium. Exam tip: connect velocity change with internal layer change.

S waves do not pass through liquid media. Exam tip: understand this as evidence for the liquid outer core.

Time differences at different stations indicate internal media. Exam tip: link travel time with layer identification.

Shadow zones form due to changed wave paths and blockage. Exam tip: connect this with core information.

Wave velocity changes between crust and mantle at Moho. Exam tip: remember Moho through seismic evidence.

Gutenberg boundary is the boundary of mantle and outer core. Exam tip: connect it with seismic wave behavior.

Lehmann boundary is considered between outer and inner core. Exam tip: remember it as an internal core boundary.

Gravity anomaly is related to mass and density. Exam tip: treat it as an indirect geophysical source.

Magnetic anomaly can indicate metallic materials inside. Exam tip: connect magnetism with core and metallic nature.

Meteorites are not direct samples of Earth's centre. Exam tip: use them for comparison with early materials.

Geothermal gradient shows relation between depth and heat. Exam tip: connect it with evidence of internal heat.

Decay of radioactive elements can release heat. Exam tip: remember it among sources of internal heat.

Weight of overlying layers increases pressure at depth. Exam tip: remember pressure as an internal physical condition.

A drilling core provides real sample. Exam tip: when real material is obtained the source is direct.

Analysis of seismic waves gives clues but not real samples. Exam tip: consider it an indirect source.

A seismograph records seismic waves. Exam tip: remember the relation between instrument and its record.

The direct answer is option A: the arrival order of waves helps us understand differences in wave velocity and the medium through which they travel. A seismogram is a recorded trace of ground vibrations. After an earthquake, different waves travel at different speeds. If one wave arrives first and another later, the time difference gives information about their velocities, the distance travelled, and changes in the material inside Earth. Waves may speed up, slow down, bend, reflect, or disappear when they meet a different layer. Option A is correct because arrival order is directly related to wave properties and the nature of the medium. Option B is wrong because crop production cost belongs to agriculture and accounting, not earthquake records. Option C is wrong because raindrop size is a weather observation unrelated to seismic arrival times. Option D is wrong because the number of towns is a population or settlement fact, not a wave measurement. For a weak learner, remember: earlier arrival usually means greater speed, provided the path is being compared correctly. Memory cue: seismogram arrival time reveals wave speed and Earth’s layers.

P and S waves are body waves that pass through the interior. Exam tip: connect body waves with internal sources.

The direct answer is A: Position of layer boundary. When an earthquake produces seismic waves, the waves travel through Earth. If a wave meets a boundary between two materials and returns, this returning is called reflection. By recording the return time and studying the wave path, scientists can estimate where the boundary lies and learn about Earth’s internal layers. Step by step: first, the wave travels downward; second, it reaches a boundary; third, part of its energy returns; fourth, instruments record the return; finally, scientists use the record to locate the boundary. Option A is correct because the reflected wave helps identify the depth or position of an internal boundary. Option B is wrong because river salinity is measured by studying water, not seismic reflection. Option C is irrelevant because air has no colour information in this process. Option D is unrelated because crop taste is a biological or agricultural matter. Memory cue: reflection from a boundary helps reveal the boundary.

Change of direction is refraction and it indicates medium change. Exam tip: link refraction with differences in layers.

Higher gravity value may indicate higher density or mass. Exam tip: connect gravity clue with density.