15. Which of the following statements are correct?(I)Hormones often regulate homeostasis through antagonistic functions.(Ⅱ) Hormones of the same cherical class usually have the same function.(Ⅲ) Hormones are secreted by specialized cells usually located in exocrine glands.(Ⅳ)Hormones are often regulated through feedback loops. (A) only II andⅢ (B) only I and IV (C) only I andⅢ (D)only Ⅲ and IV

5.Density-dependent inhibition is explained by which of the following processes? (A) As cells become more numerous, the protein kinases they produce begin to compete with each other, such that the proteins produced by one cell essentially cancel those produced by its neighbor. (B) As cells become more numerous, the cell surface proteins of one ceill contact the adjoining cells, and they signal each other to stop (C) As cells become more numerous, they begin to squeeze against each other, restricting their size. (D)As cells become more numerous, the level of waste products increases, which slows metabolism and inhibits growth.

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