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。