( )的计算原则是以承包商为某项索赔工作所支付的实际开支为根据，向业主提出费用补偿。

A．总费用法

B．修正的总费用法

C．实际费用法

D．预算费用法

By the time he was eighteen, he _________ three foreign languages.[ ]

A. learn        

B. learned          

C. has learned          

D. had learned