单项选择题
A signal indicating zone time is preceded with the letter（）.
A.L B.N C.T D.Z
单项选择题 A signal indicating Greenwich mean time would be preceded with（）.
单项选择题 A sign of thunderstorm development is a cumulus cloud（）.
单项选择题 A shipmate suffers a heart attack and stops breathing. You must（）.
