Which description best explains how a doppler radar system works?
a. it reflects waves off of objects, measuring the difference of the waves it sends and receives.
b. it sends out a set of waves where each wave is pre-programmed to detect a certain kind of object.
c. it uses an antenna to measure the angle between two or more objects in distant space.
d. it uses the doppler effect to raise the pitch of sound waves so objects in distant space can be heard.

A Doppler radar system transmits radio waves off of objects
and receives what bounces back. 

The system measures the difference between the frequency
of the waves it sends and the frequency of the waves it receives.

If the waves come back at a higher frequency than they were sent,
that means the object is moving toward the radar antenna.

If the waves come back at a lower frequency than they were sent,
that means the object is moving away from the radar antenna.

This is kind of what choice-A is trying to say.