We must decide between A and B.

我们必须在A和B之间抉择。

It is situated midway between a and b.

它位于甲乙两地的中间。

Let us make a comparison between a and b.

让我们比较一下生活在城市和生活在农村。

It's a given — there is a height difference between a and between b.

这是给定的条件-，点a点b之间，存在高度差。

Changes in lake level must be due to the net difference [between a and b].

湖水含量的变化一定是由于A和B之间的净差值。