The writer wants to add a sentence immediately before sentence 3 to connect the evidence and commentary provided in the second paragraph to the overall argument of the passage. Which of the following choices most effectively accomplishes this goal? A. While NASA astronauts are already experienced with the difficulties humans experience in outer space, these problems would be intensified on long-term missions.

B. Many people dream of space travel from a young age, but only a small number of applicants for astronaut training have the technical knowledge and physical traits needed to succeed as an astronaut.

C. A mission to Mars would entail three distinct gravitational conditions: first, Earth’s gravity; then, the low gravity of outer space; and finally, the gravity of Mars, which is about 38 percent as strong as Earth’s gravity.

D. Astronauts face extraordinary dangers when they travel into space.

E. It is important to conduct medical research under low-gravity conditions.