单项选择题
When we say that two objects have relative motion, we mean （）.
A.the distance between them remains the same
B.the distance between them is changing
C.the position between them is not changing
D.their positions to the earth remain the same
相关考题
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单项选择题
The trouble-free life of an engine indicates （）.
A.the period of time during which no trouble occurs to the engine
B.no trouble will occur to the engine all its life
C.trouble may occur freely to the engine
D.no matter what you do to the engine, no trouble will occur during this period -
单项选择题
General, the best way for replacement of lamp is （）.
A.after the point is reached at which the failures begin to increase rapidly
B.before the point is reached at which the failures begin to increase rapidly
C.before the point is reached at which the failures begin to increase slowly
D.after the point is reached at which the failures begin to increase slowly -
单项选择题
With （）, the corrosion problems are much reduced in a central cooling system.
A.no equipment in contact with sea water
B.less equipment in contact with fresh water
C.more equipment in contact with sea water
D.less equipment in contact with sea water
