Which is a role of helpful bacteria?

removing nutrients from soil

producing oxygen

healing sick animals

preventing tooth decay

Producing oxygen

Explanation:

Bacteria actually add to nutrients in soil because they decompose the organic matter such as dead bodies and such. I do not know if bacteria heal sick animals or if they prevent tooth decay rather I am sure they cause both of those.

Therefore, most plausible answer is producing oxygen because there are bacterias like cyanobacteria, or blue-green algae who go through photosynthesis and produce oxygen which is beneficial for all living beings.

if the type of the food available changes, then the frequency of beak types will change, because birds with beaks more suited to the available food will be more successful over time.

explanation:

the universal gravitation equation. since the gravitational force is directly proportional to the mass of both interacting objects, more massive objects will attract each other with a greater gravitational force. if the mass of one of the objects is doubled, then the force of gravity between them is doubled.