总账管理子系统，在凭证管理中无法完成以下（）操作。
A.凭证记账
B.出纳复核
C.凭证禁用
D.调用凭证模版
参考答案：D
We were swimming in the lake ______ suddenly the storm started. [ ]
A．when
B．while
C．because
D．before
