The loading, unloading, transshipment and transit of inward and outward mail bags are subject to Customs control and a ______ wya bill should be submitted to the Customs by the enterprise providing prviding postal service.

A．cover

B．covering

C．covered

D．haveing covered

骨的生理过程，主要体现为成骨和破骨作用以及钙和磷的代谢过程。