The conditions that will result in the formation of frost are _______ .
A.the temperature of the collecting surface is at or below freezing and small droplets of moisture are falling
B.when dew forms and the temperature is below freezing
C.temperature of the collecting surface is below the dewpoint of surrounding air and the dewpoint is colder than freezing
D.small drops of moisture falling on the collecting surface when the surrounding air temperature is at or below freezing
正确答案是C