根据英文解释,写出相应的英文单词

根据英文解释,写出相应的英文单词
1.make air,rivers,ect.dirty and dangerous
2.a word that you use to say what is possible or true when another thing happen or true
3.something bought or got as a gift to remind a person or a place
4.a railway system under the ground of a city
5.belonging to a certain place or area
6.being related with the west partof the world
7.fail to hit,catch,reach
我把2的完整版发出来
2.------（a word that you use to say what is possible or true when another thing happen or true）you press the button,the machine will start.