Question

【题目】 When and where this took place _____ still unknown.

A. is B. has

C. were D. are

Answer 1

【答案】A

【解析】

试题分析：考查系动词。句意：这个发生在何时何地还不清楚。主语从句作主语，谓语动词用单数形式，故选A。