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?

The snail carries out cellular respiration, using oxygen and releasing carbon dioxide. In sunlight, the Elodea photosynthesizes, using carbon dioxide and releasing oxygen. Each organism supplies a gas the other can use, helping both survive in the sealed tank. The correct choice is “The snail produces carbon dioxide, while the plant produces oxygen.”

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