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mitochondria (power of the cell house)

hope it somewhat helped :)

the dark variety, as they are not visible on the dark tree barks

explanation:

the menstrual cycle – higher tier. negative feedback mechanisms affect the production of hormones in the menstrual cycle. oestrogen also stimulates the release of lh (luteinising hormone) from the pituitary gland, which in turn controls the production of progesterone.