Question

which statement about the cycling of materials through a stable ecosystem is true?

radiant energy and heat are continuously recycled from animals to plants.

certain species of bacteria are responsible for cycling nitrogen from the atmosphere to plants.

carbon dioxide is continuously cycled through a stable ecosystem, but only bacteria can convert it to a usable form.

water and energy are the only materials in an ecosystem that cannot be recycled.

Explanation:

1. Analyze Option 1: Energy (radiant, heat) flows one - way (from sun to producers to consumers, lost as heat), not recycled from animals to plants. So this is false.
2. Analyze Option 2: Nitrogen - fixing bacteria convert atmospheric nitrogen ($N_2$) into forms (like ammonia) that plants can use. This is a key part of the nitrogen cycle. So this statement is true.
3. Analyze Option 3: Plants (via photosynthesis) also convert $CO_2$ to usable organic carbon, not just bacteria. So this is false.
4. Analyze Option 4: Water is recycled (water cycle), and energy is not recycled (flows one - way), but the statement misclassifies water. So this is false.

Answer:

B. Certain species of bacteria are responsible for cycling nitrogen from the atmosphere to plants. (Note: Here we assume the options are labeled A, B, C, D with the first option as A, second as B, etc. based on the standard multiple - choice format. If the original question had different labels, adjust accordingly.)