Why do presence of P waves and absence of S waves together inform about the outer core?

Correct answer: P waves can pass through liquid but S waves cannot

The direct answer is A: P waves can pass through liquid, but S waves cannot. To understand this, first learn the basic difference. P waves are primary or compressional waves. They move particles by compression and expansion along the direction of travel, so they can travel through solids, liquids, and gases. S waves are secondary or shear waves. They move particles at right angles to the direction of travel and require a rigid material whose particles can resist shearing. Liquids cannot support this kind of sideways shear, so S waves stop at a liquid layer. Step by step: earthquake waves reach the Earth’s interior; P waves continue, though they bend and slow or change speed; S waves disappear after meeting the outer core; therefore the outer core must behave as a liquid. Option A is correct and explains why the two observations together are important. Option B is wrong because neither P nor S waves is restricted only to gases. Option C reverses the real behavior: P waves do not always stop, and S waves do not pass through liquids. Option D is wrong because seismic P and S waves are mechanical waves, not light waves; light is an electromagnetic wave. Exam cue: P means push and can pass through liquid; S means shear and needs a solid.