单项选择题

Laser-dopple radar ______.

A．measures the movement of a spaceship by means of light beam
B．makes use of microwaves
C．makes use of sound waves
D．both A and B
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单项选择题
Which of the following statements is NOT true according to the passage ______.
A．accuracy in speed measurement is essential to spaceships
B．the speeds of space vehicles will vary greatly
C．inaccuracies in measurement could cause accidents in space missions
D．a bump could affect the preciseness of the laser-dopple radar
单项选择题
Implied but not stated ______.
A．Laser-dopple 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-dopple radar
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