[判断题][T AB061 4 3 1
( )当止回阀上游介质的压力低于下游介质的压力时阀门打开,实现介质的流通。
A.正确
B.错误

{{B}} Why They Came{{/B}}
Not many decisions could have been more difficult for a family to make them to say farewell to a community where it had lived for centuries, to abandon old ties and familiar landmarks, and to sail across dark seas to a strange land. Today, when mass communications tell one part of the world all about another, it is quite easy to understand how poverty or tyranny might force people to exchange an old nation for a new one. But centuries ago migration was a leap into the unknown. It was an enormous intellectual and emotional commitment. The forces that moved early immigrants to their great decision — the decision to leave their homes and begin an adventure filled with uncertainty, risk and hardship — must have been of overpowering proportions. As Oscar Handlin states, the early immigrants of America "would collide with unaccustomed problems, learn to understand alien ways and alien languages, manage to su
A. freedom of worship
B. religious intolerance
C. economic success
D. respect for the rights of man