A:Had it not been for
B:If it were not
C:If it had not been for
D:Were it not for
如图所示,磁场与线圈平面垂直,先后以速度v1和v2匀速把一矩形线圈拉出有界匀强磁场区域,v1=3v2.在先后两种情况下( )
在人类性别决定中起决定作用的是( )