When two species of paramecium, $P$ caudatum and $P$ . aurelia, are grown in separate culture dishes, each population grows rapidly and then levels off at the carrying capacity for its environment. When they are combined in one culture dish, P. aurelia survives, but $P$ . caudatum does not.
This is most likely because
(A) $P$ . caudatum suffers a mutation that prevents its survival
(B) $P$ . caudatum was attacked by a pathogenic virus or bacteria
(C) $P$ . aurelia must have evolved into a superior organism
(D) The two populations are competing with each other, and $P$ . aurelia can outcompete $P$ . caudatum
(E) $P .$ aurelia is a predator, and $P$ . caudatum is its prey