Do you know the difference water and oil?When you ____ ,what will be on the top?

题目

Do you know the difference water and oil?When you ____ ,what will be on the top?
A.mix it up B mix up it C mix them up ,水和油都是不可数名词,应该用it 来代替,为什么答案是选C啊,

答案

把这2样东西混合,故是复数