What is meant by variable expressivity in genetics?
a the product of one gene controls the expression of another gene.
b a certain percentage of individuals with a particular genotype fail to express the associated phenotype.
c a single genotype produces a range of phenotypes that differ in the degree or magnitude of expression of the particular trait specified by the gene.
d a single genotype produces multiple distinct phenotypes that affect different functions.

C A single genotype produces a range of phenotypes that differ in the degree or magnitude of expression of the particular trait specified by the gene.

Explanation:

Variable expressivity refers to the range of signs and symptoms that can occur in different people with the same genetic condition.

For example, the features of Marfan syndrome vary widely— some people have only mild symptoms (such as being tall and thin with long, slender fingers), while others also experience life-threatening complications involving the heart and blood vessels. Although the features are highly variable, most people with this disorder have a mutation in the same gene (FBN1).

answer: ice crystals in air

the water from the water body or another source of water gets evaporated as water vapor as an effect of heat on water. these water vapor rises in the upper layers of the earth atmosphere under high atmospheric pressure. the atmospheric pressure of these water molecules starts to decrease due to the effect of cold air. the invisible water vapor condenses into visible water droplets and ice crystals.these ice crystals and water droplets being less denser than the surrounding air floats on air as clouds.

these muscles maintain the blood pressure in the vessels. the smooth muscles are innervated by nerves of the autonomic nervous system. when the muscles contract, they cause vasoconstriction of the vessels and increase blood pressure. when the muscles relax, they enable vasodilation of the   blood vessels hence reduce blood pressure.