In order to look after her sick daughter, the old lady asked for a week’s ________.

A．holiday

B．vacation

C．leave

D．rest

电路如图所示，电容初始电压为零，开关在t=0时闭合，则t≥0时u（t）为（）。

A．(1-e^-0.5t)V

B．(1+e^-0.5t)V

C．(1-e^-2t)V

D．(1+e^-2t)V