— Look, the light in the living room is still on.

—Oh, I forgot _______.

A．turning it off

B．turn it off

C．to turn it off

D．having turned it off

一理想变压器的原线圈为50匝，副线圈为100匝，当原线圈接入正弦交变电流时，副线圈的输出电压为10V，则铁芯中磁通量变化率的最大值为（　　）

A．0.14Wb/S

B．0.4Wb/s

C．0.2Wb/s

D．0.28Wb/s