Question

why are bacteria needed in the nitrogen cycle?
other living things cannot use atmospheric nitrogen.
other living things cannot use the nitrogen in plants.
bacteria are the main nitrogen reservoir.
bacteria convert nitrogen gas into oxygen.

Explanation:

1. Analyze Option 1: Atmospheric nitrogen ($N_2$) is in a form that most organisms can't use directly. Nitrogen - fixing bacteria convert atmospheric nitrogen into usable forms (like ammonia) for other living things. This matches the role of bacteria in the nitrogen cycle.
2. Analyze Option 2: Other living things (like animals) can use nitrogen from plants (e.g., by eating plants and getting nitrogen - containing compounds), so this is incorrect.
3. Analyze Option 3: The main nitrogen reservoir is the atmosphere (mostly $N_2$), not bacteria, so this is incorrect.
4. Analyze Option 4: Bacteria do not convert nitrogen gas into oxygen; they convert nitrogen gas into nitrogen - containing compounds for biological use, so this is incorrect.

Answer:

A. Other living things cannot use atmospheric nitrogen.