ADD After Spin Class

I stare at the identical lockers, the idiot beat of the dance music from class still pounding in my head. I cannot remember which one is mine, and I can't remember how the locks work, and I can't remember my four-digit combination.

I try this one, entering variations of the numbers I'm pretty sure I put in, to no avail, then that one, also with no luck. 

"You have to push 'C,' then the number, then the button with the little key on it," the woman sitting on the bench offers in an attempt to be helpful.