A．laser-doppler radar is more accurate than microwave radar
B．microwave radar may not ensure absolute precision in measuring varying speeds
C．the light-beam radar can measure only slow speeds with accuracy
D．microwave radar operates at a higher frequency than laser-doppler radar

A．Pretending to be somebody else.
B．Following somebody from in front.
C．Following somebody from behind.
D．Standing in front of a shop window.