当机长发出“撤离”指令时，乘务员的第一个口令是（）。

A.“系好安全带“

B.“紧迫用力“

C.“解开安全带“

D.“到这边来“

It is only when you nearly lose someone ____________ you fully realize how much you value him.  

A．do

B．then

C．will

D．that