They discovered the cause of the fire accidentally. They discovered it .A. fortunatelyB. b

问题补充：

They discovered the cause of the fire accidentally. They discovered it .

A. fortunately

B. by chance

C. all of a sudden

D. on purpose

答案：

A. fortunately

(答案→)B. by chance

C. all of a sudden

D. on purpose

解析：考查短语辨析。Fortunately 幸运地，by chance 偶然地，all of a sudden 突然地，on purpose 故意地，句意：他们无意发现了大火的起因。他们偶然发现的。所以选B。