What have given mankind a new viewpoint on the earth and its neighbors in space

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.

A．That people dreamed of leaving their home planet and exploring other worlds became reality.
B．After the Apollo program, the emphasis in piloted missions shifted to short-duration spaceflight.
C．Space exploration is a great challenge to human beings and will be achieved through generation's work.
D．Today efforts focus on keeping people healthy during space missions and in increasing the cost of sending satellites into orbit.