Gravity is the single factor and plays the largest role in the maintaining of a person's adequate bone strength. This question is missing the beginning that is very important to have to correctly answer the question. The missing part of the question is "Mechanical stress on weightbearing bones plays a large part in bone remodeling." With that being written, now to explain why gravity is the answer. While astronauts are in space, they must limit their muscle weakness. They will do several weight-bearing exercises every day to simulate the gravity that they experience on earth.
