英语语法求助：Suppose you have to do one of the two,which do you pr

英语语法求助：Suppose you have to do one of the two,which do you prefer?
我在一本英语口语的书上看到这句话：
Suppose you have to do one of the two,which do you prefer?
后来我查了一下语法书,发现Suppose翻译成“假如”的时候,
可以用作虚拟语气,等同于If.
例如：
If it is fine,we will go picnicking.
= Supposing it is fine,we will go picnicking.
我的问题是：
which do you prefer?这句话应该把do换成will,因为这是虚拟语气.
那为什么书上不写成which will you prefer?