Question

a lack of exercise could
cause bones to become longer.
cause bones to become thicker.
cause bones to store more calcium.
cause bones to lose their medullary cavity.
result in porous and weak bones.

Explanation:

Exercise stimulates bone - building cells. Lack of it reduces bone density. Bones become porous and weak, a condition related to osteoporosis. Bones don't become longer, thicker, store more calcium or lose medullary cavity due to lack of exercise.

Answer:

result in porous and weak bones.