3. The absorptive effectiveness of the small intestine is enhanced by increasing the surface area by
A villi.
C. digestive juices
B. enzymes.
D. elasticity of intestine,​

A villi.

Explanation:

Small intestines have finger-like extensions that basically increase the surface area for nutrients to attach and get transported inside the bloodstream or cells.

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).