为自己加油

The two indentical fueling adapters are used for _______ (燃油喷口)connection, and are designed to prevent tuel in the fueling manifold from ________(防止回流) through the adapters.

A.fueling nozzle,flowing back;
B.fueling manifold, return;
C.fueling adapter,  flowing back;
D.fueling valve, flowing back;