The author suggests that Americans suffer from hypertension as a result of______.

A．Members of a species differ from one another.
B．differences between individuals are inherited.
C．Two organisms with identical genes may grow to different sizes.
D．More organisms are born than live to reproduce.

A．control the direction of artificial selection
B．control the direction of natural selection
C．make new genes in artificial selection
D．make new genes in natural selection