After coupling, the air shut-off valves at the back of the first trailer should be _________.

Shut-off valves (also called cut-out cocks) are used in the service and supply air-lines at the back of trailers used to tow other trailers. You must check that all shut-off valves are in the open position except the ones at the back of the last trailer, which must be closed.
(According to CDL Handbook)