(ac) comes from "acuoso", aqueous in Spanish. I used it all over my Olympiad exams, without any penalization, because the entire exam was in Spanish.
Those zinc equations were for an assignment. In Spanish again.
I was taught to use (ac), as you can see. This is what every future Olympic will be taught, so I'll try to show this discrepancy, and an example of what happens when they use (ac) instead of (aq). I'll use (aq) as long as I don't forget when posting here, also.