How the big forest fire has remains to be a mystery.
A. turned up
B. broken out
C. taken place
D. broken up
网友回答
A. turned up
(答案→)B. broken out
C. taken place
D. broken up
解析：句意，这场森林大火是如何爆发的仍然是个谜。turned up出现 broken out指（火灾、战争）等突然爆发 taken place发生 broken up分裂，根据题意，故选B。