Discuss the importance of oxygen to all living organisms, and describe how three different organisms—human, flatworm, and fern—obtain the oxygen they need.

the answer is d (apex) good luck : ))

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).