“Cool" is a word with many meanings．Its old meaning is used to express a temperature that is a little bit cold．As the world has changed，the word has had many different meanings．
“Cool"can be used to express feelings of interest in almost everything When you see a famous car in the street，maybe you will say，“It's cool." You may think，“He's so cool," when you see your favorite footballer．
We all maximize（扩大） the meaning of “cool". You can use it instead of many words such as “new" or "surprising"．Here's an interesting story we can use to show the way the word is used．A teacher asked her students to write about the waterfall（瀑布）they had visited．On one student's paper was just the one sentence, “It's so cool．Maybe he thought it was the best way to show what he saw and felt. But the story also shows a lack of words．Without “cool”，some people have no words to show the same meaning．So it is quite important to keep some credibility（可信性）．Can you think of many other words that make your life as colorful as the word "cool"？ I can．And I think they are also very cool．
在酸性溶液中易被还原成
的氧化性依次减弱。下列反应在水溶液中不可能发生的是 ( )
