18. Prolactin stimulates mammary gland growth and development in mammals and regulates salt and water balance in freshwater fish. Many scientists think that this wide range of functions indicates which of the following? (A) Prolactin is a nonspecific hormone. (B) Prolactin is an evolutionary conserved hormone. (C) Prolactin has a unique mechanism for eliciting its effects. (D) Prolactin interacts with many different receptor molecules. (E) Prolactin is derived from two separate sources.