They are part of an organ system
when working out you want to make sure you can meet the task at hand. go little by little until you are able to meet your goal. so to start i would rule out D.
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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I would go with all of the above
