—I knocked but nobody answered the door.

----They _____asleep then and didn’t hear you.

A．must be

B．should be

C．must have been

D．should have been

按《UCP600》，凡信用证未注明可撤销或不可撤销字样的，应视为（）。这项修改，对国际贸易的促进，产生了巨大的积极作用。