什么是助磨剂？对助磨剂有什么要求？简述助磨剂的助磨机理。
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IS—LM模型是说明（）与（）同时达到均衡时， 国内生产总值与利率决定的模型。
Which statement about satellite navigation is TRUE? （）
A. While a fix can be generated by signals received from two satellites，three satellites are necessary for reliable accuracy
B. The satellites are in equatorial orbits around the earth about 60°of longitude apart
C. The satellite navigation system determines a fix by measuring the doppler shift of the radio signals from the satellite
D. The ship's receiver cannot begin processing data until the antenna locks onto the satellite and starts continuous tracking
组合开关的通断能力较低，故不可用来分断（）电流。
A. 负荷
B. 额定
C. 正常
D. 故障
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