【题目】Exactly______ the potato was introduced into Europe is uncertain, but it was probably around 1565.
A.whether
B.why
C.when
D.how

【答案】C
【解析】句意：准确的说，土豆是什么时候被传到欧洲的还不确定，但是可能在1565年左右。此处when引导主语从句并在从句中作主语，选C。