Which of the following is an example of top-down processes? (a) killer whales in the North Pacific have reduced the number of sea otters, thereby causing an increase in the number of sea urchins and a decline in kelp populations (b) experiments at Hubbard Brook have addressed the hydrology of forests and associated aquatic ecosystems (c) one tree provides food for thousands of insects (d) in African grasslands, the number of herbivores is greater than the number of carnivores (e) the bioaccumulation of DDT in a Long Island salt marsh.