Two cities were both 100 miles away from the center of an earthquake, but the vibrations reached one city before they reached the other.

Why might the seismic waves have traveled faster to one city than the other?
A.
One city was smaller than the other.

B.
One city was quieter that the other.

C.
It is impossible for waves to travel at different speeds.

D.
They were traveling through different types of rocks.