Actually Cs are average, D is like ok but not good then it goes E, F and G, then B is really good, A is extremely good and A* is perfect.
Honestly your mum is just over-reacting and she's just trying to think about you and how she might want you to go to a great college, trust me, she most likely is, but it's still and over-reaction and you need to talk to her about it.
Also British grades are changing from 1-9 where 3 is a C.
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