如图所示,四边形的通电闭合线框abcd处在垂直线框平面的匀强磁场中,它受到磁场的合力( )
A:竖直向上
B:垂直于ab斜向上
C:垂直于bc斜向下
D:为零
Why not send an email to her instead of ____ (reply) to her letter?