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
单项选择题 The trouble-free life of an engine indicates （）.
单项选择题 General, the best way for replacement of lamp is （）.
单项选择题 With （）, the corrosion problems are much reduced in a central cooling system.
