Vitamin D increases the efficiency of the intestine to absorb calcium and phosphorus
from food in order to mineralize the bones in the body. It also increases the activity of bone
cells that make and lay down bone matrix. The bone matrix is like the frame of a building.
If the body has adequate amounts of calcium and phosphorus, “they” are incorporated into
the bone matrix, and the result is a strong, healthy skeleton. Our skin can make vitamin D
when it is exposed to sunlight. However, most people living in North America do not get
enough sunlight in the winter to make adequate amounts of the vitamin.
Young children who do not get enough calcium and vitamin D are unable to properly
mineralize the bone matrix. Consequently, when gravity pushes on the skeleton, it causes
the typical bowing of the legs seen in a child with the disease called rickets.
In adults, a deficiency in both calcium and vitamin D will increase the risk of bone
fracture. Vitamin D is necessary to increase the body's ability to absorb calcium. If the body
does not have enough vitamin D, it can absorb only 10 to 15 percent of the calcium it receives.
If the bloodstream does not have enough calcium, it will draw it out of the bones, which
causes osteoporosis. With osteoporosis, the bones break down as bone cells called osteoclasts
dissolve the matrix and release calcium from the bones. A vitamin D deficiency will increase
the severity of the disease because it increases the number of holes in the bones.

The word “they”in paragraph 1 refers to

(A) vitamin D and calcium
(B) bones
(c) bone cells
(D) calcium and phosphorus