Answer:
Answer is D
Explanation:
As the heart pumps blood, a series of valves open and close tightly. These valves ensure that blood flows in only one direction, preventing backflow. The tricuspid valve is situated between the right atrium and right ventricle. The pulmonary valve is between the right ventricle and the pulmonary artery.
Hunger and thirst are considered to be internal stimuli.
Hope that helped you.
Answer:
I believe the answer is A. Produce their own food.
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I'm pretty sure it is refraction <span />