Which of the following is true about extinction?

A. There are many causes of extinctions, and an animal or plant may go extinct due to a combination of these factors.
B. Introduction of new alien species helps preserve biodiversity and slow extinction.
C. Commercial hunting and fishing are careful not to deplete animal reserves.
D. Overexploitation by hunting and fishing is the foremost threat to tropical animals and plants.
E. Without human activity, all species would survive, and there would be no extinctions.