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（）.
单项选择题 A（）is created by a pressure gradient or slope in the water level.
单项选择题 （）is the most widespread type likely to be encountered at sea and is caused by relatively warm air being cooled by flowing over a cooler sea surface.
