suddenly occurred to me I had to attend an important meeting in Shanghai the next day.A.

问题补充：

suddenly occurred to me I had to attend an important meeting in Shanghai the next day.

A. It; which

B. What; that

C. What; which

D. It; that

答案：

A. It; which

B. What; that

C. What; which

(答案→)D. It; that

解析：考察it作为形式主语。本句指的it是形式主语，真正的主语是后面的that引导的主语从句，that在主语从句中不充当任何成分，只起引导和连接作用。本句中还有固定句式It occur to sb that…某人突然想起某事；句意：我突然想起我明天要去上海参加一个重要的会议。故D正确。