答案：C
质量为m的小球，用一条绳子系在竖直平面内做圆周运动，小球到达最高点时的速度为v，到达最低点时的速度变为，则两位置处绳子所受的张力之差是[ ]
A.6mg
B.5mg
C.4mg
D．2mg
( )Prefixes modify the meaning of the stem and also change the part of speech of the original word．
，则两位置处绳子所受的张力之差是[ ]