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?
O While NASA astronauts are already experienced with the difficulties humans experience in outer space, these problems would be intensified on long-term missions.
O 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.
O 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.
O Astronauts face extraordinary dangers when they travel into space.
O It is important to conduct medical research under low-gravity conditions.