Biotic factors are living parts of an ecosystem. An example of a biotic factor is A. a trout swimming in a river
В. bacteria growing on bread
C. moss growing in a tree
D. all of the above

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).