Auto fill options

I can't find online how to deal with this fill problem.

I have a column where I want to drag a fill. Usually that's no problem. But the equation has some cell values in it that I DON'T want it to change, but it does no matter what I do. How can I have some values increment (A1, A2, A3... etc) but others don't (C5 is always C5)

Example of what I want to happen after using the auto fill from

A1+C5
A2+C5

Creates

Cell Row 1: A1+C5
Cell Row 2: A2+C5
Cell Row 3: A3+C5
Cell Row 4: A4+C5
Cell Row 5: A5+C5
...
Cell Row n: An+C5