3) which of the following is (are) true? a) hormones regulate cellular functions, and generally negative feedback regulates hormone levels. b) the circulating level of a hormone is held constant through a series of positive feedback loops. c) both lipid-soluble hormones and water-soluble hormones bind to intracellular protein receptors. d) the ducts of endocrine organs release their contents into the bloodstream. e) only a and c are true.