I want to buy a pair of shose ,but i do not know A which one

这里主要还是看搭配名字的量词,一般没有量词情况：i want to buy a book,but i do not know which ... 这里就用one就可以了
另外,如果说句子是there are a lot of books on the shelf, some are fiction books, some are detective books, and others are... but i do not know... 根据上下文,这里就需要用which kind of book了.