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

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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。