"The volleyball must belong to Mary" means_________

A．The volleyball must be Mary

B．Mary has a volleyball

C．The volleyball must be Mary's

D．That can't be Mary's volleyball

如图所示，半径为r的圆筒，绕竖直中心轴OO′转动，小物块a靠在圆筒的内壁上，它与圆筒的动摩擦因数为μ，现要使A不下落，则圆筒转动的角速度ω至少为______．