So quick A* question

So A* requires 90 UMS at A2, so does that mean as long as one gets 90 UMS at A2, A b at AS still makes it possible but c doesn't?

To score the A*, you need to score at least 90% over the A2 units and at least 80% over the full A-Level. Technically, you can score a C at AS but still score an A* overall if you score >90UMS (but you'd need to work very, very, very hard!).