Identifying Non-Rational Numbers

Consider the following numbers:

1. $$rac{8}{3}$$

2. $$rac{3}{0}$$

3. $$rac{4}{-2}$$

4. $$rac{5}{5}$$

5. $$rac{ ext{5}}{ ext{x}}$$, where $$ ext{x}$$ is irrational.

Out of these numbers, which one is not a rational number?