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  3. A new kind of radar has been developed for spaceship travelers. A working laboratory model of a new system of radar that makes u

A new kind of radar has been developed for spaceship travelers. A working laboratory model of a new system of radar that makes u

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问题     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 used microwaves.
    The model has shown that this radar system, known as laser-doppler 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 ten 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 (百万兆) of cycles 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-doppler radar is especially valuable in space missions because ______. ( )
选项 A、it is one thousand times more precise than microwave radar
B、it provides precise measures of the velocity of space vehicles
C、it is a new system of radar
D、it can measure the movement of a spaceship less accurately than a microwave radar
答案B
解析
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