speak to ur form tutor/ relevant subject teachers and ask how they feel the course would suit you- it's difficulty etc.
why are you doing an extra subject? are you dropping one of your current ones? apparently some unis prefer you just to concentrate on your original 3- i'm only doing biology because that is my weakness in psychology + i want to improve.
on the plus side, biology is a subject that (i find) is very logical and easy to understand
What really put the nail in biology's illogical coffin was when I saw the word HEDGEROW in the ecology module!