Which of the following is an example of a closed system? A) An estuary where fresh and ocean water mix. B) A chemical reaction within deep ocean organisms allowing them to glow. C) A living single-celled organism such as phytoplankton. D) A river that flows into the ocean.

C) a living single-celled organism such as phytoplankton

just go with your heart buddy

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.