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

问题补充：

—When did the Shanghai Expo open?

—May 1st, .

A. In

B. On

C. At

D. By

答案：

A. In

(答案→)B. On

C. At

D. By

解析：在时间前只要有天的前用介词on，本题中有个1号，故本题选B。