When it comes to blood circulation, a good and effective blood circulation is one that provides the body with a lot of nutrients - so what we want is for example an increased blood circulation to the body, but we don't want for example a decreased blood flow to some organs - those organs would then not receive the necessary nutrition. Healthy lifestyle provides good things to the body - so the answer is D, since it's the only good thing. Decreased area in blood vessels would again diminish the nutrition to some organs and a high blood pressure is also not good - it carries a risk of some diseases.
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The skull protects the brain from any direct damage and the vertebrae helps keep the nerves in the spine in place and protected from damage. So the central nervous system is protected by the vertebrae and the skull.
