Which of the following is true regarding physical changes during young adulthood? a. Individuals with dark skin show skin aging earlier than their counterparts with white skin. b. The heart muscle...

Which of the following is true regarding physical changes during young adulthood?

a. Individuals with dark skin show skin aging earlier than their counterparts with white skin.

b. The heart muscle starts to become rigid.

c. The immune system increases its production of T cells and B cells.

d. There are subtle changes in the respiratory system including an increase in vital capacity.