It hasn't been made clear the new underground line is to be open to traffic.A. whenB. tha

问题补充：

It hasn been made clear the new underground line is to be open to traffic.

A. when

B. that

C. what

D. where

答案：

(答案→)A. when

B. that

C. what

D. where

解析：A 考察名词性从句。这是when引导的主语从句，it是形式主语。句意：新的地铁线路何时开放还不清楚。故A正确。