单项选择题
A.control the pressure of the oxygen as it enters the body
B.allow the wearer to manually give himself oxygen
C.release excess heat which would otherwise cause the bottle to explode
D.allow exhaled gases to pass outside the bottle
单项选择题 The extension of the after part of the keel in a single-screw vessel upon which the stern post rests is called the（）.
单项选择题 The exhaust pipe must be gas tight throughout its entire length otherwise（）.
单项选择题 The connected joints of pipe，usually made of three joints of pipe approximately 90 feet long，racked in the derrick when making a trip are called a（）.