Answer:
2.A substance formed by the chemical union of two or more elements
Explanation:
The context clues say that water molecule is made up of TWO hydrogen atoms so that was a context clue.Then common sense is that it is science and that was the only definition that have something to do with science
The correct answer is A. During this excerpt you can realize how important it is for the narrator the air, but not just only the air, the things that it involves, and the example is that the air the same for all, there is the social aspect that the narrator wants you to see and to understand the consequences of the things we do affects us all