不是的

Medicare part A, hospitalization coverage, you will have to sign up

Medicare part B, medical treatment coverage, you will have to sign up

Medicare part C, Medicare Advantage, alternative private insurance plans you can use instead of A/B/D.  

This is just your usual health insurance company, e.g. UnitedHealth, Aethna etc.  If you buy Medicare Advantage plan from them, they will take your share of funding from medicare funding. 

Medicare part D, drug coverage

Medicare part E/F/G... Medigap plans, covering gaps from part A/B/D.   AARP is famous for selling this kind of plan.