It was April 29, 2011 _______ Prince William and Kate Middleton walked into the palace hall of the wedding ceremony. [ ]

A. that

B. when

C. since

D. before

为了满足厂用电电压水平的需要，高压启动／备用变压器通常装有（）装置。

A.有载调压；

B.在线稳压；

C.电压监测；

D.电压平衡。