Multiple choice:

Question

For a natural ecosystem to be self-sustaining,
many compounds must be:
1. converted into new forms of energy.
2. cycled between organisms and the environment.
3. permanently removed from the environment.
4. changed into fossil fuels such as oil and
coal

Answer 1

2: cycled between organisms and the environment

The compounds must be taken in by consumption, and then recycled back through decomposition.

Answer 2

cycled between organisms and the environment