问题补充：
How the fire in the dancing hall started a mystery.
A. to remain
B. remains
C. is remaining
D. is remained
答案：
A. to remain
(答案→)B. remains
C. is remaining
D. is remained
解析：句意:舞厅的大火如何开始的仍然是一个迷。这里How the fire in the dancing hall started是主语从句，从句做主语谓语动词用单数，remain这里是系动词，没有进行时和被动形式，故选B。
