[单项选择题] The loading, unloading, transshipment and transit of inward and outward mail bags are ( )

[单项选择题] The loading, unloading, transshipment and transit of inward and outward mail bags are ( ) Customs control and a covering way bill should be submitted to the Customs by the enterprise providing postal service.

A．subject to

B．upon

C．into

D．with