A new kind of radar has been developed for spaceship travelers. A working laboratory model of a new system of radar that makes use of a beam of light is said to be ten thousand times more accurate than the best comparable system of radar that uses microwaves.
The model has shown that this radar system, known as laser-dopple radar, can measure with absolute precision speeds varying from spaceship orbital injection(进入)velocities(速度)of five miles per second down to virtual stops-speeds of less than one-thousandth of an inch per seconD.According to the scientists who are developing this system, such fine measures of velocity are of prime importance in space missions. In a rendezvous(对接) between two spaceships, or in a landing approach by a vehicle onto an orbiting space station, a bump could rip open a ship's skin, or a nudge could knock the station out of its orbit.
The light-beam radar, which operates at a frequency of trillions of cycles(百万兆) per second, could easily detect and measure the movement of a vehicle edging up to a satellite space station. A control system using so precise a signal as this would allow a huge vehicle to dock at a space station as lightly as a feather.
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

单项选择题The stars twinkled in the transparent clarity of the sky. （ )

单项选择题1.第一台计算机是哪一年发明的（） A、1950年 B、1956年 C、1946年 D、1949年

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未分类题按权责发牛制计算，该企业1月份的收入为( )元。D.12000