Acid Rain

By definition, acid rain is acidic. Since we know that acid has a pH of less than $7$ , we can rewrite the solutions as double inequalities, where $x$ are the possible pH values, such that the first satisfies $6.5 < x < 7$ , the second satisfies $7 ≤ x < 7$ , and the third and fourth are $4.5 ≤ x < 7$ and $7 < x < 10$ .

Using the process of elimination, we see that the last option makes the pH of $x$ alkaline, which is not acidic. The second option has $x = 7$ , but that would be neutral (like water, which is not acidic). Finally, the first option does not include $x$ where it is equal to $1, 2,$ or $3$ , which are obviously very acidic but not included under the inequality. It follows that the third option, $x ≤ 4.5$ is the only one that is correct.