18. It was because of English girlfriend believe it or not his English improved so grea

问题补充：

18. It was because of English girlfriend, believe it or not, his English improved so greatly.

A. which

B. as

C. that

D. what

答案：

A. which

B. as

(答案→)C. that

D. what

解析：为强调句型，被强调成分为 because of English girlfriend。由于句中插入了believe it or not这一成分，致使许多同学误认为这是一个非限制性定语从句。