DNA Pairing *isssue*

So i understood the basic idea of DNA sequence .its the process of working out the order of the four bases A,C,G,T in a strand .And the given examples on the challenge also makes sense except the second one.

I dont understand , i mean can we put base from the four DNA bases even tho the base is not present in the sequence. Clearly TTGAG dont have C in it but still it prints ["G","C"] . HOW ?? some explanation would be great.
[I have not started coding for the algo yet tho , im just trying to understand the problem first, so didn’t provide any link]
@camperextraordinaire wanna take a look mate ! :upside_down_face:

Your code needs to contain some sort of lookup scheme. For example, if you look up A your lookup will reply T. Then you put that in arrays as required by the challenge.

TTGAG --> [["T", _], ["T", _], [ "G", _ ], ["A", _ ], ["G",_ ]]
You fill in the blanks.