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In the past, the study of space was mainly carried out by astronomers using telescopes. However, astronomers sometimes had it wrong. Meanwhile, apart from the desire to conquer the world around, humans also want more space to accommodate the world's increasing population. Owing to the developments in space technology, humans have made spectacular explorations and discoveries of the space. The explorations can be (23)_________either by unmanned robotic probes or by human spaceflights. Undeniably, those achievements have been made quickly as a result of the "space race" by the Soviet Union and (24)_______United States. Within more than two decades, from the beginning of 1950s to 1970s, the two nations launched many of the milestones, from launching living beings into space to launching flybys to observe planets. (25)________, the biggest achievement of the 21st century has been the cooperation in launching the International Space Station, which serves as space environment research laboratory for experiments in many(26)_________. It is also suited for the testing of spacecraft systems and equipment required for missions to the Moon and Mars. Hopefully, in the near future, space technological advancements will soon allow us to (27)  ________ out habitable space environment.

23. A. conducted                    B. made                                  C. carried                                D. practiced
24. A. some                             B. a                                          C. an                                      D. the
25. A. Moreover                      B. However                               C. But                                     D. So
26. A. countries                       B. ways                                    C. fields                                  D states
27. A. find                               B. look                                     C. discover                              D. know