31、B possible life on Mars.
32、B Less than one year.
33、C When it got near Mars.
34、D Scientists used different measurement systems.
35、C They were tired from working long hours.
３６－４０：CDACA
41. C find warmth and food
42. D north America
43. A lead young cranes on their first trip south
44. A 1200 miles
45. B learn to migrate on their own