Why are primary waves from an earthquake focus recorded first?

Correct answer: Because their velocity is higher

The direct answer is B: primary waves have greater velocity. P-waves, or primary waves, are compressional seismic waves. They push and pull particles in the direction in which the wave travels and generally move faster than S-waves. The calculation idea is simple: for the same earthquake distance, the wave with greater speed takes less time, so it is recorded first. Thus P-waves arrive before S-waves, and the time difference can help locate an earthquake. Option B is correct. Option A is wrong because being slow would make a wave arrive later. Option C is wrong because P-waves travel through solids and liquids, not only water. Option D is wrong because P-waves can travel through the Earth and may be detected at the surface by seismographs; they are not defined by failing to reach the surface. Do not confuse “primary” with “most important”; here it means first arrival. Memory cue: P means primary and prompt—the prompt, faster wave arrives first.