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

【答案】D
【解析】句意：马铃薯被引入欧洲的时间不确定，但很可能是1565年。A. whether是否；B. why 为什么；C. how怎样；D. when什么时候。 Exactly ______ the potato was introduced into Europe 是主语从句，根据后面的时间可知，前面是在说时间，故选D。