An English question: Which one of these is grammatically correct?

Question

A) I don't think so, many quit being football players after a decade or so.

B) I don't think so, many quit being a football player after a decade or so.

C) I don't think so, many quit after a decade of being football players.

D) I don't think so, many quit after a decade of being a football player.

Answer 1

I would make it two sentences. Then they would all be okay. 

Note, "after a decade or so", in A and B, suggests more time than "a decade" in C and D

Answer 2

The grammar is ok, but you need a semi-colon between the two clauses. D also has an agreement problem- "many" should be "players".

I don't think so; many quit after a decade of playing football.

Answer 3

All your sentences are grammatically correct.