What are the four other primary sources of supplemental coverage?

According to the article list below, (a) What are the four other primary sources of supplemental coverage? (b) Characterize the 10% of beneficiaries who only receive basic Medicare coverage. (c) How might supplemental coverage create a moral hazard problem? (d) What arguments do Vladeck and others make in defense of supplemental coverage

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a) employer sponsored plans; individually purchased plans; Medicaid and supplemental coverage from other public sources

b)poor but not poor enough to qualify for Medicaid)

c) the supplemental coverage helps insulate Medicare beneficiaries from the cost of their care ( reduced levels of cost sharing,) therefore encourage beneficiaries use of additional services in response to reduced levels of cost sharing,

d)restricting access to decent supplemental coverage would surely transfer billions of dollars in costs to a segment of society remarkably ill equipped to absorb it and increase growing inequality in income, resources, and access to services in the US population.