6. Which of the following statements regarding aquaporins is correct? (1) Aquaporins are water-channel proteins. (2) Aquaporins specifically increase the permeability of biomembranes to water- soluble salts. (3) The level of aquaporin 2 is rate-limiting for water transport by the kidney and is essential for resorption of water in the kidney. (A) (1) and (2) (B) (1) and (3) (C) (2) and (3) (D) All of above are correct (E) None of above is correct