It's not like that for the other subjects.

Chemistry has just "Chemistry", good for two college semesters.

Physics has four classes! There isn't an "A" or "B" or "AB", but there is a "C"! You can choose "Physics C Mechanics" and "Physics C Electricity and Magnetism", each worth a college semester. (you can do both) Alternately you can choose "Physics 1" and "Physics 2", which are each worth a semester for non-science non-engineering majors.

Economics has both "Macroeconomics" and "Microeconomics". It's not "A" and "AB", nor is it "A" and "B".