How the big forest fire has remains to be a mystery.A. turned upB. broken outC. taken plac

问题补充：

How the big forest fire has remains to be a mystery.

A. turned up

B. broken out

C. taken place

D. broken up

答案：

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。