Why would a testcross be important in genetics?

Fossils found in strata reveal that a) innate drive to complexity of life b) unused body parts decrease in size c) older strata carry fossils that differ greatly from living organisms d) geologic changes occur quickly on earth

Adigestive system that is a series of tubes beginning at the mouth and ending at the anus is a digestive system.

Veins have a much lower blood pressure than arteries. which of these prevents backflow of blood in veins? a. pressure applied by the heart b. one–way valves in veins c. thin muscular walls of veins