A: When did the Shanghai Expo open? B: May 1st .A. InB. OnC. AtD. By

问题补充：

A: When did the Shanghai Expo open?

B: May 1st, .

A. In

B. On

C. At

D. By

答案：

A. In

(答案→)B. On

C. At

D. By

解析：在具体的一天前加介词on，故选B