There was a big forest fire last week but nobody knows how it.A. took placeB. turned upC.

问题补充：

There was a big forest fire last week, but nobody knows how it.

A. took place

B. turned up

C. broke out

D. broke up

答案：

A. took place

B. turned up

(答案→)C. broke out

D. broke up

解析：句意，上周发生了一场大森林火灾，但是没有人知道它是怎样爆发的。took place 发生 turned up 出现 broke out （战争、火灾等）突然发生，爆发 broke up 分裂，根据题意，故选C。