Why does the climate in the southern hemisphere tend to be milder than the northern

Because the Southern Hemisphere has more ocean and much less land. As you may know water heats up and cools down more slowly than land.

b. the carbon cycle ensures living things have a source of hydrogen

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).