7.如图所示,一长为I的轻质硬杆下方悬挂一质量-|||-为m的小球,小球以O点为圆心可在竖直平面内-|||-做半径为l的圆周运动,如果要使小球运动到最低-|||-点处的速度v1是在最高点速度的5倍则需要满足-|||-的条件是 ()-|||-o 1-|||-,-|||-i-|||-、-|||-A (upsilon )_(1)=sqrt (dfrac {{z)^1}(6)}-|||-B (upsilon )_(1)=5sqrt (dfrac {81)(6)}-|||-C (upsilon )_(1)=5sqrt ({e)^1}-|||-D (upsilon )_(1)=sqrt ({e)^1}