问题
选择题
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
答案：B
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。