(1) Space flights to other planets have long been a fixture of science fiction, and in 2017 the fiction appeared to come closer to reality when the National Aeronautics and Space Administration (NASA) announced it was prioritizing eventual piloted missions to Mars and other planets. (2) It may be too soon to start planning your Martian vacation, however: there are good reasons to doubt that humans will be able to undertake long-distance space flights. (3) For example, the body begins to lose muscle mass and bone density in low-gravity environments because the pressure of Earth’s atmosphere is removed. (4) Astronauts struggle to walk after returning from a six-month space mission; the effects of a three-year flight and a stay on Mars would be far greater. (5) Similarly, humans have a natural cycle of wakefulness and sleep attuned to daytime and nighttime on Earth. (6) Anyone who has experienced jet lag can attest to the mental and physical effects of disrupting this cycle, but we can only imagine the consequences of disrupting it for years on end.

(7) It remains to be seen whether these problems can be diminished enough for long space voyages, and it is true that monitoring, exercise, and technological fixes like sunlight-simulating LED lights have shown promise. (8) Other issues may be more troublesome. (9) Space missions are stressful, requiring long hours of repetitive activity performed in small, cramped spaces. (10) The stress lowers immune response even as outer space conditions cause microbes in astronauts’ bodies to mutate, potentially resulting in dangerous diseases. (11) Cosmic rays pose another health concern: a 2018 study from Georgetown University Medical Center found that prolonged exposure to cosmic radiation could lead to stomach and colon cancers. (12) Unless scientists can develop effective means of fighting these health risks, long-distance space travel may be too dangerous.

(13) With NASA’s attention directed to the problems of long space missions, it is certainly possible that other nations will invest in similar research. (14) But it is also possible that further research will turn up new problems that have not even been identified yet.

The writer wants to add a concluding sentence that brings the argument to a unified end by summing up the main claim of the passage and returning to a theme from the first paragraph. Which of the following choices, if added after sentence 14, best accomplishes this goal?

a. It is thus likely that for many years to come, the only way for humans to have an experience exploring distant planets will be by opening a book or going to the movies rather than hopping on a rocket.
b. Ultimately, these problems are likely to be overcome, because the efforts of NASA are now being supplemented by those of private spaceflight companies.
c. It is nonetheless important to remain optimistic about reaching Mars—the first step to achieving a goal is to envision a successful outcome.
d. Some argue that NASA’s space shuttle program took attention away from more ambitious goals and that preparing to go to Mars is a better direction for the agency.
e. Sending robots rather than humans eliminates many of the dangers and difficulties, but there is something uniquely inspiring about humans traveling into space.