第一部分 单项选择题（每题1分，共15题）
1. Exactly ______ the potato was introduced into Europe is uncertain, but it was probably around 1565.
A. whether B. why C. how D. when
2. --- Why are you so late?
--- I was on half the way when it ____ to me that I had left my notebook home, so I had to fetch it.
A. hit B. occurred C. happened D. reminded
