There are many problems 1______ space travel. The first and greatest of them is gravity. If you let your pencil drop to the floor, you can see gravity in 2______ Everything is held down to the earth 3______ magnetic force. A rocket must go al least two thousand and five hundred miles an hour 4______ anyone beyond the gravity of the 5______ into space.
Another problem is the strain that a person is 6______ to when a rocket leaves the ground. Anything that is not moving 7______ to resist movement. As the rocket leaves the ground, it pushes upward violently, and the person in the nose cone is pushed back against the chair. During this thrust, gravity 8______ a force on the body 9______ nine times its 10______ force. 11______ out of the earth's gravity, an astronaut is affected by another problem weightlessness. Here, if a pencil drops, it does not fall. If a glass of water is turned 12______ , the water will not fall out. Our bodies, which are 13______ to gravity, tend to become upset in weightless conditions. Recent long flights have shown that the body needs special exercise in a 14______.
Astronauts could also be 15______ by boredom and loneliness. Some of them might have to sit in their spaceships for months with 16______ to do and no one 17______ Space trips to 18______ planets or the nearest stars might take many years. It is possible that some trips might 19______ take a life time. So future astronauts must be trained to 20______ long periods of inactivity and solitude.
1
A.connected to B.connected with C.connecting to D.connecting with正确答案B
