In a sealed tank is a large sprig of Elodea and a snail. The tank is kept for several weeks on a sunny window sill. Both the plant and snail live and appear to be healthy. Why does this happen? a. The snail produces oxygen, and the plant produces carbon dioxide. b. Both the snail and the plant produce oxygen. c. The snail produces carbon dioxide, while the plant produces oxygen. d. Both the snail and plant produce carbon dioxide.