I’ll never forget the day the great scientist came to our school and gave us a speech. A.

问题补充：

I’ll never forget the day the great scientist came to our school and gave us a speech.

A. that

B. which

C. when

D. where

答案：

A. that

B. which

(答案→)C. when

D. where

解析：定语从句的用法。句意：我从来不会忘记这位伟大的科学家来我们学校给我们做演讲的这一天。从句中有主语，也有宾语，可判断先行词做状语，表示时间用when。that和which是关系代词，做主语或宾语；where做状语时表示地点。故选C。