is certain though is that “the curse of the mummy” remains a riddle to this day.A. AsB

问题补充：

is certain, though, is that “the curse of the mummy” remains a riddle to this day.

A. As

B. Which

C. It

D. What

答案：

A. As

B. Which

C. It

(答案→)D. What

解析：考察从句连词的选择。根据句子结构可知，该句为主语从句，从句中缺少is certain的主语，故选择what。