Nobody's mentioned decision maths yet. But I don't blame them because when I did my AS in it I found it increidbly tedious. At the end of the day it is just following algorithms, but they have such daft names for them I constantly confused them all with each other.
Was about to mention that. I did the C1, C2 and S1 modules this year. Not sure whether I should do S2 or D1... choices, I have to pick them in about 10hours time.

