1 . 令人惊喜的是，直到20世纪70年代，现代人才有人知道它们。
________________ no one in modern times knew about them until the 1970s.

3 . 但是在网上花太多时间是不健康的，并且很难集中精力在生活中的其他事情上。
But __________ too much time online __________ and makes it __________ other things in life.

4 . 对于你们这一代青少年来说，喜欢电脑游戏和网络世界并不罕见。
It is __________ for teenagers of your generation __________ computer games and the online world.

5 . _____________________ (很难想象一个没有金属的世界。) Different metals have different uses, for example, steel is used in cars, and iron is used in electrical equipment.