Car 1 Moves Toward Car 2 with Speed V

Multiple Choice

Car 1 moves toward car 2 with speed v. An observer in car 2 measures the frequency of the sound emitted by car 1's horn to be f. Now, car 1 remains stationary while car 2 moves toward car 1 with speed v. The observer in car 2 now measures the frequency of car 1's horn to be f'. The relationship between the observed frequencies is

A) f < f'.
B) f = f'.
C) f = 2 f'.
D) f = 1/2 f'.
E) f > f'.