问题
选择题
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
D
题目分析：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。