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

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A（）is created by a pressure gradient or slope in the water level.

A.gradient current
B.density current
C.swift current
D.torrent current

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.

A.FRONTAL FOG
B.MIXING FOG
C.ADVECTION FOG
D.RADIATION FOG

单项选择题（）is a type of fog occurring close to the sea surface when the air is dry and cold-probably at least 9℃ below the sea surface temperature.

A.MIXING FOG
B.ADVECTION FOG
C.RADIATION FOG
D.ARCTIC SEA SMOKE

单项选择题（）forms over land，most frequently during autumn and winter over low-lying land，especially if it is damp and marshy and in valleys on quiet nights with clear skies.

A.FRONTAL FOG
B.MIXING FOG
C.ADVECTION FOG
D.RADIATION FOG