It because they give us oxygen and without oxygen we would all die from oxygen deprivation.
the process that gives us oxygen is photosynthesis
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Each person pushing should feel the same force pushing back
If you are implying one these choices not needed for survival it would be parents only when you can survive on your own (I'm not that sure but I hope this helped the slightest but)
It form the contraction and relaxation of muscle
They both reproductive Pacific cells
