D. A keystone species existence keeps an ecosystem in balance.
People breathe out Carbon Dioxide and plants use that Carbon Dioxide to carry out photosynthesis. Plants release oxygen as a by- product that people use for cellular respiration.
Answer:
Hope it helps
Explanation:
Earth's amazing gaseous atmosphere is responsible for making life possible on this, the third planet from the Sun. Our atmosphere contains water vapor which helps to moderate our daily temperatures. Our atmosphere contains 21% oxygen, which is necessary for us to breathe, 78% nitrogen,
A, because its still water, just in a different state.
