Why does the oceanic crust continuously subduct

When an oceanic plate converges with a continental plate, the oceanic crust will always subduct under the continental crust. As the oceanic crust is naturally denser, this melting leads to heat being transferred upwards and uplifting the crust, eventually developing into a volcano.

"a.) well–developed vascular system."

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).