A:bc>0
B:b﹣d>0
C:b+c>0
D:|a|>|d|
甲、乙两物体都做匀速圆周运动,其质量之比为1∶2 ,转动半径之比为1∶2 ,在相等时间里甲转过60°,乙转过45°,则它们所受外力的合力之比为( )
—I want to win the first prize in the coming school English competition.
—________.