Sources of information

Reflection is the clue of waves returning from a boundary. Exam tip: study reflection with internal boundaries.

Ocean floor rocks show crustal material but do not give core samples. Exam tip: remember source limitation.

Volcanic material is useful but not representative of all deep layers. Exam tip: write limitation along with usefulness.

Mining is a direct source but its depth is limited. Exam tip: remember the limitation of direct sources.

Artificial waves give clues but not real samples. Exam tip: treat it as a controlled indirect method.

Both anomalies are indirect clues so comparison is necessary. Exam tip: write combined evidence for final conclusion.

Magnetism indicates metallic nature and S waves indicate liquid state. Exam tip: remember combined evidence for core.

Analysis of rock samples reveals minerals and elements. Exam tip: remember sample analysis for chemical composition.

Sources like mining and drilling are useful but do not reach deep parts. Exam tip: write both use and limitation.

Both direct and indirect evidence must be combined for deep interior. Exam tip: treat comparison of sources as scientific method.

Records from different stations are useful for comparison. Exam tip: connect network data with reliable evidence.

P wave velocity is affected by nature of medium. Exam tip: connect decrease in velocity with state of medium.

Reflection occurs when a wave reaches a boundary and returns into the medium from which it came. The wave may change its direction at the boundary, but the main feature in this question is that it comes back. Seismic waves can be reflected at boundaries between layers inside the Earth, and their reflected signals help scientists study the Earth's internal structure.

Option B, reflection, is correct. Refraction is different: it is the bending or change in speed and direction of a wave as it enters a medium or layer with different properties. Evaporation is the change of liquid into vapour, and weathering is the breakdown of rocks at or near the surface. Since the question says the wave returns from a boundary, the process is reflection.

The direct answer is C: rain gauge and the liquid state of the core. Scientists use several indirect and direct sources to study Earth’s interior. Gravity measurements can reveal density clues because underground masses affect the gravitational field. Magnetic studies can provide clues about electrically conducting or metallic material and Earth’s magnetic field. Seismic waves reveal internal boundaries and layers because their speed and paths change in different materials. A rain gauge measures the amount of rainfall at a place; it is a meteorological instrument and has no direct role in determining whether the core is liquid. Option A is a correct match because gravity variations can indicate density differences. Option B is a reasonable correct match because magnetic observations are related to conducting, metallic material and the generation of Earth’s magnetic field. Option C is incorrect because rainfall measurement cannot establish the physical state of the core. Option D is correct because seismic waves help identify the arrangement and properties of internal layers. The solving method is to ask what each source measures and whether that measurement can logically provide the claimed information. Memory cue: rain gauge belongs to weather; gravity, magnetism and seismic waves belong to Earth-interior clues.

Mining provides direct information about Earth’s interior because rocks obtained from mines can be studied. Its major limitation is limited depth: mining cannot reach the mantle or the core. Seismic waves provide indirect evidence about the interior, not actual rock samples. Exam tip: remember mining and drilling as direct sources, while seismic and gravity measurements are indirect sources.

Scientists combine direct and indirect evidence to draw conclusions. Exam tip: treat combined analysis as the main method.

Meteorites are not direct samples obtained from Earth's mines. Exam tip: connect them with comparison of early materials.

Mining is limited in depth while waves give clues from deep parts. Exam tip: write seismic source for deep study.

P waves pass through many media while S waves do not pass through liquid. Exam tip: identify state from medium capacity.

Drilling gives samples and boreholes give temperature data. Exam tip: treat it as a limited direct source.

Indirect sources are verified with other evidence. Exam tip: combined evidence makes answers stronger.

Seismic analysis identifies internal layers through wave behavior. Exam tip: check relation between method and purpose.

Depth temperature and pressure make direct study difficult. Exam tip: write importance of indirect sources for this reason.

Scientific study makes conclusions based on evidence. Exam tip: connect conclusions with matching of sources.

A is correct. Sheet wash is runoff flowing over a slope in a thin, spread-out layer. Small irregularities guide some of the water into narrow paths; the concentrated flow gains enough force to cut shallow channels called rills. If they enlarge and join, rills can develop into gullies.