The percentage of water in the air is the
a. absolute humidity.
b. comparative humidity.
c. virtual humidity.
d. relative humidity.

answer is absolute humidity

explanation is  

HUMIDITY

Have you ever stepped outside and a humid day and had your hair start to frizz? Humidity is a measure of how much water vapor is in the air. On very humid days, water vapor causes hair to become longer. The extra length causes frizziness. Water vapor enters the atmosphere during the evaporation step of the water cycle. Warm air carrying water vapor rises and then cools. As it cools, the water vapor condenses onto salt, dust, and other particles in the air forming clouds. Clouds are an important part of weather. They reflect incoming solar radiation, help trap heat on the earth's surface, and create precipitation. When water droplets in a cloud grow too large and heavy to float in the air, they fall as precipitation. If the air around the cloud is freezing, the water droplets fall as snow. Water droplets which begin as rain, then freeze as they fall through a layer of cold air form sleet. Hail and freezing rain are other types of precipitation which can fall from clouds.

Without water vapor, there would be no clouds and no precipitation. The earth would be colder, too, since water vapor is one of the greenhouse gases which trap heat in the atmosphere. Although it can make a hot day miserable and sticky, humidity is an important part of earth's weather.

he is very short, and he can only reach halfway up on the elevator buttons. but on a rainy day, he has his umbrella with him, and he uses it to poke the button at the top.

genetic variation can be caused by mutation (which can create entirely new alleles in a population), random mating, random fertilization, and recombination between homologous chromosomes during meiosis (which reshuffles alleles within an organism's offspring).