which是有选择范围的.What没有.
比如你到书店中想买一本英语词典,看到有《牛津》,有《朗文》还有《新英汉》,你不知道该买哪一本,就说:I couldn't decide which dictionary I should buy.
你到水果店里,看到各种各样的水果,你都不知道买什么好了.可以说：I don't know what fruit I should buy.