问题
选择题
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
答案：C
题目分析：考查主语从句的连接词。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。