Bio Engineer Archive

Does anyone have any idea how this works. When used, it gives you another interface. It gives you 4 pairs: for example, AC, GC, TA, AT, and then says find the mission pairs. It also gives you something like Sample: for example, CAC. Then, below it is a large list of letters (either one or two letters) that you must pick and find a "matching" pair.

Okay, I'm guessing most of you don't understand what I just said, because it's hard to explain. But, I was wondering for the people who have used them before:

How does it work? What's the trick to it? How do you tell which item to select?