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