The answer is correlation
The majority of mammals (including humans) utilize a double circulatory system. ... Double circulatory systems are important because they ensure that we are giving our tissues and muscles blood full of oxygen, instead of a mixture of oxygenated and deoxygenated blood.
The right option is; C. They are controlled by more than one gene
Traits such as skin color, human height and eye colour are known as polygenic traits. These traits have wide variation throughout the human population and are controlled by two or more genes. For example the human height is controlled by at least three genes with six alleles. The quantities of the genes are usually large but often have a little effect.
Capillaries are the smallest of all blood vessels.
A tube that extends from a bacterium
