回答下列各题： A new kind of radar has been developed for spaceship travelers．A working l

aboratory mod-el of a new system of radar that makes useof a beam of light is said to be ten thousand timesmore accurate than the bestcomparable system of radar that used microwaves． The model has shown that this radar system，known as laser—dopplerradar，can measurewith absolute precision speeds varyingfrom spaceship orbital injection(进入)velocities(速度)of five miles per second down to virtual stops--speeds of less thanone ten thousandth ofall inch per second．According tothe scientists who are developing this system，such finemeas-ures of velocity are of prime importance in space missions．In a rendezvous(对接)betweentwo spaceships，or in a landing approach by a vehicle onto an orbiting space station，a bumpcould rip open a ship’S skin，or anudge could knock the station out of its orbit． The light-beam radar，which operates at afrequency of trillions(百万兆)of cycles persecond，could easily detect and measure the movement of a vehicle edging upto a satellitespace station．A control system using SOprecise a signal as this would allow a huge vehicle todock at a space stationas lighfly as a feather． Laser—doppler radar_______．

A.makes use of microwaves

B.measures the movements ofa spaceship by means of light beam

C.makes use of sound waves

D.Both A and B