59–62  In any group of elderly people, it is common to see some women who are unable to stand completely upright. Their backs and heads are bent forward slightly, as if they were carrying a heavy load. This is usually a sign of a bone problem that has developed over a number of years, a problem called osteoporosis.Osteoporosis literally means porous bones—bones that are filled with tiny holes. Our bones are in a constant state of change, continuously broken down and rebuilt through the body’s chemical processes. Two kinds of cells are important in these processes: osteoclasts and osteoblasts. Osteoclasts renew the old bone by dissolving or reabsorbing it; this leaves an empty space. Osteoblasts then fill this empty space with new bone.When the rate of bone renewal doesn’t equal the rate of breakdown, bone loss results. When the bone loss becomes advanced, it is called osteoporosis. The bones don’t actually change shape with osteoporosis; rather, they become porous, and therefore are more likely to break. Often the first sign of osteoporosis is a bone fracture following a minor fall or accident.
59. What is the main subject of the article?
(A) Treatment for osteoporosis
(B) Exercise and osteoporosis
(C) Prevention of osteoporosis
(D) Description of osteoporosis