The best surface to run on is one that's a. as hard as possible so that it adds more stability to your foot. b. uneven so that the tendons and ligaments in the ankles and knees are strengthened. c. hilly so that you can improve your cardiovascular endurance. d. smooth and soft so that it can cushion the stresses of running.

d) center of the mid-ocean ridge

give brainleysest

sinserly,

steckh986

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).