A seismic P-wave with a frequency of 5.0Hz travels through rock at a speed of 5.0×103ms−1. A seismograph is located 250km from the earthquake’s epicenter. When the first crest of the P-wave arrives at the seismograph, how many additional crests are still propagating between the epicenter and the seismograph?
a) 250
b) 499
c) 249
d) 50