Why Is Health Insurance Referred to as a "Third-Party Payer

Why is health insurance referred to as a "third-party payer"?

A) Because a third party, the payer, pays a provider (second party) for the patient (first party) .
B) Because it was originally required only when a husband and wife have a child, the third party.
C) Because it only pays for one-third of the expenses.
D) Because it is the third option for payment, following the patient and the government.