Osteoporosis result from which of the followings?

Osteoporosis result from which of the followings? a.        A decline of osteoblast activity while osteoclast activity continues at expected levelsb.        A decline of osteoclast activity while osteoblast activity continues at expected levelsc.        An increase in osteocyte activity while osteoblast activity continues at expected levelsd.        An increase in osteocyte activity while osteoclast activity continues at expected levels