The kidneys secrete ADH in response to dehydration.

A. Monozygotic; dizygotic: Monozygotic twins are identical twins that result from the division of a single fertilized egg. Dizygotic twins, or fraternal twins, result from the fertilization of two separate eggs.

B. Dizygotic; monozygotic: This is the reverse of the correct order. Dizygotic twins come from two eggs, and monozygotic twins come from one egg.

C. Monozygotic; identical: Monozygotic twins are identical, but this choice does not mention dizygotic twins, which makes it incomplete.

D. Dizygotic; nonidentical: While dizygotic twins are nonidentical, this choice does not properly define the types of twins produced by a single egg (monozygotic).

A. Penetrate the vagina: The acrosome does not play a role in penetrating the vagina. Its function is specifically related to the sperm's ability to penetrate the ovum, making this option incorrect.

B. Dissolve the stickiness of the semen: While enzymes do play a role in semen liquefaction, this is not the function of the acrosome. The acrosome is primarily involved in the process of fertilization, not semen modification.

C. Dissolve the mucosa of the uterus: The acrosome is not involved in dissolving the mucosa of the uterus. Its function is related to penetrating the barriers surrounding the ovum.

D. Penetrate barriers surrounding the ovum: The acrosome contains enzymes that break down the outer layers of the ovum (such as the zona pellucida), allowing the sperm to penetrate and fertilize the egg. This is the correct answer.