Combustion is the burning of a substance. It could be complete or could be incomplete. It changes the form of carbon and so it plays it's role in cycle
Lack of vitamin D has been linked to increased risk for autoimmune diseases.
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Frogs do have lungs but they are small and rarely used, frogs mostly take in moisture through the skin in process called cutaneous gas exchange.
Answer:
The return of blood to the heart is assisted by the action of the skeletal- muscle pump. As muscles move, they squeeze the veins running through them. Veins contain a series of one-way valves, and they are squeezed, blood is pushed through the valves, which then close to prevent backflow.