A．ought
B．drought
C．thought
D．bought

A．accuracy ill speed measurement is essential to spaceships
B．the speeds of space vehicles will vary
C．inaccuracies in measurement could cause accidents in space missions
D．a bump could affect the preciseness of the laser-doppler radar

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