What is concerning us greatly is the workers held up in that area yesterday will be set f

问题补充：

What is concerning us greatly is the workers held up in that area yesterday will be set free.

A. that

B. what

C. when

D. which

答案：

A. that

B. what

(答案→)C. when

D. which

解析：考查表语从句。根据句意可知应该是何时。句意：最让我担心的事情是在那个地区被绑架的工人何时被释放。