A. While NASA astronauts are already experienced with the difficulties humans experience in outer space, these problems would be intensified on long-term missions.
Explanation:
Answer A
Correct. The second paragraph provides evidence of the many harmful effects of long-term space travel on the human body, including loss of “muscle mass and bone density” and disruption of the “natural cycle of wakefulness and sleep.” This sentence would effectively lead into this discussion and establish the role of the second paragraph in the overall argument of the passage.
