Which of the following is true about loss of bone strength in middle adulthood?
a) loss in bone density usually begins in the early twenties and accelerates in the thirties.
b) environmental factors do little to slow bone loss in postmenopausal women.
c) loss of bone strength causes the disks of the spinal column to collapse.
d) by the end of middle adulthood, women have lost 70 percent of their bone density.