I could have turned to Mr. Smith for help, but he ______ too busy then.

A．is

B．was

C．should be

D．has been

如图所示，带铁芯线圈置于竖直悬挂的闭合铝框右侧，与线圈连接的导线abcd围成的区域内有水平向里的变化磁场．下图哪种变化的磁场可使铝框向右靠近（   ）