The frequency of sound will decrease when; a) the source approaches a stationary observer
b) the observer approaches a stationary source
c) the source recedes from a stationary observer
d) the observer recedes from a stationary source
e) both a and b
f) both c and d
g) both a and d
h) both b and c
i) none of these

The Doppler effect says that if the source of a sound wave and the observer are moving further apart, the frequency of the sound as the observer hears it will decrease. This can be caused by the source moving away from the observer or the observer moving away from the source. Both C and D will cause the decrease in frequency.

Choice F

c. magnitude and direction