A. immune system
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The best answer for the blood vessels' valves would be these are veins that would then help in preventing the back flow of blood because the walls have small muscle to pump the blood back to the heart.Hope this is the answer.
The correct term to fill in the blank is activation-synthesis. The sentence above summarizes the activation-synthesis view of dreaming. It is when the dreams encountered are results when cerebral cortex tries to make something out of the neural activity that is happening in the less-advanced part of the brain.
