What is the purpose of using backup rings with O-rings in hydraulic systems above 1,500 PSl?

A、Preventinternalandexternalleakageofallmovingpartswithinahydraulicsystem.
B、Provideasealbetweentwopartsofaunitwhichmoveinrelationtoeachother.
C、Preventhighpressurefromextrudingthesea)betweenthemovingandstationarypart.