单项选择题
A line of clouds，sharp changes in wind direction，and squalls are most frequently associated with a(n)（）.
A.occluded front B.warm front C.cold front D.warm sector
单项选择题 A hygrometer is a device used for determining（）.
单项选择题 A generally circular low pressure area is called a(n)（）.
单项选择题 A frontal thunderstorm is caused by（）.
