A stationary listener stands on a straight roadway. Four different situations involving identical car horns (rest frequency 450 Hz) are described below. In which situation will the listener hear the largest beat frequency?
A. Two cars drive in the same direction past the listener, one at 15 m/s and the other at 25 m/s.
B. Two cars drive toward the listener from opposite directions, each at 20 m/s.
C. One car drives toward the listener at 30 m/s while a second, already past the listener, drives away at 30 m/s.
D. Two cars, both moving away from the listener in opposite directions, travel at 35 m/s and 40 m/s, respectively.
a) Situation D
b) Situation C
c) Situation A
d) Situation B