 A public Part C Medicare Advantage beneficiary must first sign up for both Part A and Part B of Medicare in order to choose Part C. 

From a beneficiary's point of view, there are several key differences between Original Medicare and Part C. For example, Medicare Advantage plans include an annual out of pocket (OOP) spend limit, critical financial protection against the costs of catastrophic illness or accident.That is, under Part C plans, there is a limit on how much a beneficiary will have to spend annually OOP.That amount is unlimited in Medicare Parts A and B, which can cause financial ruin and incents almost everyone on just Parts A and B to make other financial protection arrangements, typically by adding some kind of private supplemental indemnity insurance (and even those policies often do not provide absolute 100% financial protection because they only cover what Parts A and B cover to the limits of Parts A and B).