A) An increase in the concentration of glucose in the blood stimulates the pancrease to secrete insulin, a hormone that lowers blood glucose concentration.

B) A high concentration of carbon dioxide in the blood causes deeper, more rapid breathing, which expels carbon dioxide.

C) Stimulation of a nerve cell causes sodium ions to leak into the cell, and the sodium influx triggers the inward leaking of even more sodium. (This is positive feedback because the trigger (increased sodium) triggers an increase in sodium into the nerve.)

D) The body's production of red blood cells, which transport oxygen from the lungs to other organs, is stimulated by a low concentration of oxygen.

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