单项选择题
A sign of thunderstorm development is a cumulus cloud（）.
A.darkening，growing in size and forming an anvil top B.that shows extensive vertical development C.creating cold downdrafts that are felt on the ground D.All of the above
单项选择题 A shipmate suffers a heart attack and stops breathing. You must（）.
单项选择题 A ship having continuous closely spaced transverse strength members is（）.
单项选择题 A shepherd’s crook is used to（）.
