Which conjunction - and, but, or so - best completes each sentence

Questions 17-20. Which conjunction - and, but, or so - best completes each sentence?

17.  For thousands of years, people have been interested in space, ______ it was only in the 1950s that technology made it possible for people to build rockets.

a. and                  b. but                    c. so

18. At the end of the 1950s and the beginning of the 1960s, the Soviet Union launched a satellite into space, then animals, ______ finally a person - Yuri Gagarin.

a. and               b. but                c. so

19. In the 1960s, the United States of America was concerned that it was losing the "Space Race," ______ it invested a lot of money and time in the space program.

a. and                b. but               c. so

20. In the 1960s, America sent several astronauts into space, ______ then finally managed to send a three-person team to the moon.

a. and               b. but                    c. so