They discovered the cause of the fire accidentally. They discovered it .
A. fortunately
B. by chance
C. all of a sudden
D. on purpose
网友回答
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。