Each course is worth 40 credits, usually made up of two different classes worth 20 credits each. So we need to do 120 credits each year, but over years 1 and 2 we need to do a minimum of 120 credits in arts subjects. For example if I chose to do: Psychology, Sociology and History in first year that would add up to 120 credits BUT only 40 credits of arts subjects. In year 2, if I carried on with Psychology and History and then chose Philosophy this would add up to 120 credits and also satisfy the minimum of 120 credits in arts subjects over years 1 and 2
EDIT: So as long as you've done a minimum of 120 credits of arts subjects by the end of year 2 then all is fine, that's why you can do a social science subject
( but I think the way it works out you can only take a class from another college for 1 year because of the way it works out, as we need 120 credits in Arts) I hope I've not confused you even more!