The class was held hostage by a hooded gunman.
 

全班学生被一名蒙面持枪歹徒挟为人质。
 

2.reckon 认为
 

Some people reckon that space exploration is a sheer waste of time and money while a majority of other people believe that it is of great importance.
 

有些人认为太空探索纯属浪费时间和金钱，但是大多数人认为太空探索有着非常重要的意义。
 

3.bounce off 弹开，反弹
 

Short sound waves bounce off even small objects.
 

短声波即使遇到小物体都会产生回音。
 

4.upshot 结果，结局
 

The upshot could be a much bigger bubble in the future.
 

结果可能是日后出现更大的泡沫。