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?（）
单项选择题 Conventional anchors are least likely to hold in a bottom consisting of（）.
单项选择题 Control of fire should be addressed（）.
