The *heart* is the main organ of the circulatory system.
The circulatory system consists of three independent systems that work together: the heart (cardiovascular), lungs (pulmonary), and arteries, veins, coronary and portal vessels (systemic). The system is responsible for the flow of blood, nutrients, oxygen and other gases, and as well as hormones to and from cells.
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The layer of the Earth's atmosphere where most planes fly is the stratosphere.
✔ A. Stratosphere
O B. Mesosphere
O C. Thermosphere
O D. Troposphere
When a candle burns, energy is released in the form of light and heat.
Candles are used to provide light because when they burn, the energy which is released takes the form of light. Similarly, because fire is involved in the process, heat is given off as well.