Dacron sails，when not in use，may be damaged if（）.

单项选择题Cyclones that have warm sectors usually move（）.

A.westerly
B.parallel to the isobars in the warm sector
C.toward the nearest high pressure area
D.faster than the accompanying cold front

单项选择题Cumulus clouds that have undergone vertical development and have become cumulonimbus in form，indicate（）.

A.clearing weather
B.that a warm front has passed
C.probable thunderstorm activity
D.an approaching hurricane or typhoon

单项选择题Cumulonimbus clouds are formed by（）.

A.vertical air movements
B.heavy rainstorms
C.horizontal air movements
D.any movement of moist air

单项选择题Corrosive liquids and acids should have which kind of label?（）

A.Skull and crossbones
B.Yellow and white
C.Red and white
D.White and black

单项选择题Conventional anchors are least likely to hold in a bottom consisting of（）.

A.soft clay
B.hard mud
C.very soft mud
D.sand