--When did the World Exposition(世博会) in Shanghai begin?The Expo began May 1 .A. i

问题补充：

--When did the World Exposition(世博会) in Shanghai begin?

--The Expo began May 1, .

A. in

B. on

C. at

D. by

答案：

A. in

(答案→)B. on

C. at

D. by

解析：考查介词用法。在具体一天的前，用介词on，句意：世博会在上海什么时候开始的？世博会是在五月1号开始的。故选B