What can your group do _____ the talent show? [ ]
A. to
B. of
C. at
D. for
答案：C
如图所示S1与S2分别是半径为r1和r2的同心圆环，磁感应强度为B的匀强磁场方向与环面垂直，范围以S1为边界，则穿过环S1的磁通量为______，穿过环S2的磁通量为______．
