The reason why Malaysia Airline Flight370 disappeared remained .
A. look into
B. to be looked into
C. looking into
D. to look into
网友回答
A. look into
(答案→)B. to be looked into
C. looking into
D. to look into
解析：句意：为什么马航370航班失联的原因还有待调查。这里使用了remain to be done“仍然有待被做”，使用不定式的被动语态，所以选B。