Questions 6 and 7 are based on the following news item. At the end of the news item, you will be given I0 seconds to answer each question.
Now listen to the news. According to the news, when did America’ s first encounter with a campus massacre occur

A．Six Stages for Repairing Sam’s Bicycle.
B．Possible Ways to Problem-solving.
C．Necessities of Problem Analysis.
D．Suggestions for Analyzing a Problem.

A．The challenge was developing rockets powerful enough and reliable enough to boost a satellite into orbit.
B．The challenge was building the satellites themselves.
C．Engineers also had to build tracking stations to maintain radio communications with these artificial "moons" as they circled the planet.
D．The development of rockets and satellites.