單項(xiàng)選擇題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


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1.單項(xiàng)選擇題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

2.單項(xiàng)選擇題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

3.單項(xiàng)選擇題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

5.單項(xiàng)選擇題You increased the （） to 150 tons that’s why the fuel oil （）.

A.pumped speed；came out
B.pumping rate；overflowed
C.flowing rate；leaked out
D.flowed speed；spilled out