Read the following passage and mark the letter A, B, C, or D on your answer sheet to indicate the correct answer to each of the questions.

  In the 1960s, the first human explorations of space began. Today scientists often debate the future of manned space flight. Should we aim for the moon again, or for Mars? It always comes down to concerns about the length and expense of such journeys. Mars is a much more difficult challenge than the moon. Mars is so far away that is world take about two years for a human crew to complete the round trip, including time spent to the planet’s surface. On the other hand, we know the moon can be visited in a fraction of that time, as was proven by the Apollo 11 mission in 1969. Distance poses other dilemmas. A popular movie, Apollo 13, tells the exciting story of how NASA managed to rescue the astronauts of the ill-fated Apollo 13 mission to the moon. With the tremendous distance between Earth and Mars, emergency retrievals like this would be impossible. The moon seems more feasible from this point of view.   As with most projects, the crux of the matter is economics. In fact, NASA has estimated that a manned flight to Mars would cost five hundred billion dollars! Critics argue that Mars is too far and the moon is too useless to us to justify any manned space flights at all. Nevertheless, should we choose to go ahead with space travel, it seems that returning to the moon is a much more likely proposition than Mars.     

The term “debate” in paragraph 1 is closest in meaning to __________.