Geologists Use Explosives to Map the Subterranean

Multiple Choice

Geologists use explosives to map the subterranean.The times for the reflected waves to arrive at a monitoring station can tell a great deal about the composition of the rock structures.Suppose the time it takes for a wave that is reflected from the interface of (1) and (2) is 2 s,and the time for another wave that is reflected between (2) and (3) is 2.5 s and the speed of the wave in layer (1) is 6 km/s and in layer (2) is 4 km/s,how thick is layer (2) ?

A) 2 km
B) 1 km
C) 3 km
D) 6 km
E) 8 km