Question

He is rather difficult to make friends with，but the friendship of his，once ________, is more true than any other．

1. A.
   gained
2. B.
   to gain
3. C.
   having gained
4. D.
   gain

Answer 1

A
解析：
once可引导时间状语从句表条件，意思是“一旦……”.此空填gained，其前省略了it is.在时间、条件、让步状语从句中，如果从句的主语和主句的主语相同，从句中谓语动词义含有be的某一种形式，可以省略从句中的主语或be动词.答案为A．例如：Once seen，it will never be forgotten．一旦见到，就永远不会忘掉.The research is so designed that once begun nothing can be done to change it．调查是预先计划的，一旦开始就无法改变.