A large basin cut into the shore，closed off by a caisson，and used for dry docking of ships is known as a（）.

A.a 2-second blast every 20 seconds
B.a 4-second blast every 20 seconds
C.an 8-second blast every 30 seconds
D.a 10-second blast every 30 seconds

A.turn off all lights that interfere with the navigation lights
B.turn on all available working lights on deck
C.ring the bell rapidly for five seconds once a minute
D.display two black diamonds in a vertical line during the day