He is rather difficult to make friends with but the friendship of his is truer than t

问题补充：

He is rather difficult to make friends with, but the friendship of his, , is truer than that of anybody else’s.

A. while gained

B. when to gain

C. after gaining

D. once gained

答案：

A. while gained

B. when to gain

C. after gaining

(答案→)D. once gained

解析：句意:和他交朋友相当困难,但是他的友谊,一旦获得了,就比其他任何人的都真实.当主从句的主语一致时，可以省略从句的主语和be动词，once gained=once his friendship is gained，选D。