Sources of information

Drilling core gives a direct sample and seismic waves give deep clues. Exam tip: write roles of both sources separately.

Mine temperature indicates internal heat but has limited scope. Exam tip: remember both use and limitation.

Velocity and direction changes indicate difference in internal medium. Exam tip: read both clues together.

P waves can pass through liquid but S waves cannot. Exam tip: this is a key clue for liquid outer core.

Gravity indicates density and seismic velocity indicates medium properties. Exam tip: use combined indirect evidence for conclusions.

Meteorites help in composition comparison but do not come directly from Earth's interior. Exam tip: write them as indirect evidence.

Moho is the crust mantle boundary where wave velocity changes. Exam tip: identify it through seismic velocity.

Gutenberg boundary lies between mantle and outer core. Exam tip: connect it with S wave blockage and P wave change.

P wave velocity and path are affected by medium nature. Exam tip: connect velocity and refraction together.

Lehmann boundary is linked with the boundary of outer and inner core. Exam tip: understand it as a boundary within the core.

Shadow zone forms from wave behavior and indicates core nature. Exam tip: do not treat it as surface shadow.

Direct answer: Option A is the best answer. A magnetic anomaly is a difference in the magnetic signal measured at one place compared with the expected signal. It can provide an indirect clue about magnetic or metallic minerals and the distribution of materials, but it does not by itself reveal whether the entire core is solid, liquid or divided into different layers. Scientists use seismic waves for that question. P-waves pass through solids and liquids, whereas S-waves do not pass through liquids; their behaviour helps infer the state of the core. Option A correctly says that magnetic evidence is limited and seismic evidence is needed. Option B is wrong because a magnetic anomaly does not measure rainfall. Option C is wrong because it is not a physical sample from the core. Option D is wrong because Earth does have a magnetic field. Exam cue: magnetic data give indirect material clues; seismic waves reveal internal state.

Volcanic materials give useful clues but are not direct core samples. Exam tip: write limitation of the source.

Higher mean density is a clue of dense internal materials. Exam tip: connect it with core composition.

Conflicting indirect clues should be compared with other evidence. Exam tip: do not write possible inference as final truth.

The geothermal gradient is the increase in temperature as depth below Earth’s surface increases. It is an observed relationship or pattern, not itself a material producing heat. Radioactive decay is a process in which unstable elements release energy, and it can contribute to Earth’s internal heat. Keeping the observation separate from its possible cause is important.

Option C is correct because it describes the two ideas accurately: the geothermal gradient shows temperature increase with depth, while radioactive decay may be one source of that heat. The remaining choices wrongly describe both terms as population sources, rain gauges, river sources, or crop sources. Those descriptions have no connection with Earth’s internal thermal structure.

Rock sample gives real material while waves help infer structure. Exam tip: separate sample and clue.

P (primary) waves travel faster than S (secondary) waves. Therefore, after leaving the earthquake focus, P waves reach a recording station first and appear first on the seismogram. S waves cannot travel through liquids, so they cannot be faster in a liquid medium. Exam tip: the time gap between P-wave and S-wave arrivals is used to estimate the distance to an earthquake.

Reflection is related to return and refraction to change in direction. Exam tip: remember difference between both processes.

Direct answer: Option B, ocean floor rock and oceanic crust. A source is the evidence we study, while scope tells us the part of Earth about which that evidence gives information. Rocks collected from or studied on the ocean floor help us understand the nature, composition and structure of the oceanic crust. Step by step: first identify the source as ocean-floor rock; next connect that source with the layer where it is found; therefore the correct scope is oceanic crust. Option A is wrong because a rain gauge measures rainfall and does not provide evidence about the inner core. Option B is correct because ocean-floor rocks are directly related to oceanic crust. Option C is wrong because a census gives population information, not evidence about the mantle. Option D is wrong because a crop report concerns agriculture, while the Moho is a geological boundary requiring seismic and rock evidence. Memory cue: match the evidence with the Earth layer it can actually represent; ocean-floor rock points to oceanic crust, not the core.

A local direct sample provides information only about rocks or materials from that particular place and limited depth. Earth’s interior is vast and variable, so one local sample may not represent the entire interior. Option D is incorrect because direct samples can reveal rock properties; their limitation is lack of broad representativeness. Exam tip: before generalising from a source, check its spatial and depth coverage.

Absence of S waves indicates liquid state and magnetic field gives metallic clue. Exam tip: remember combined evidence.

Artificial waves are generated and their behavior gives clues. Exam tip: write it as an indirect method.

Lava gives limited direct clues and waves give depth clues. Exam tip: combine sources by their scope.

S waves do not travel in liquid and travel in solid medium. Exam tip: understand meaning of both presence and absence.