Waves from a Radio Station with a Wavelength of 644

Multiple Choice

Waves from a radio station with a wavelength of 644 m arrive at a home receiver a distance 50 km away from the transmitter by two paths.One is a direct-line path and the second by reflection from a mountain directly behind the receiver.What is the minimum distance between the mountain and receiver such that destructive interference occurs at the location of the listener? Assume no phase change on reflection.

A) 161 m
B) 322 m
C) 483 m
D) 644 m
E) 1290 m