开凿平硐、斜井和立井时，自井口到坚硬岩层之间的井巷必须砌碹，并向坚硬岩层内至少延深（）m。

Lifeboat hatchets should be（）.

A.kept in a locker

B.secured at each end of the boat with a lanyard

C.kept next to the boat coxswain

D.kept in the emergency locker on the ship and brought to the lifeboat when needed