Identifying Non-Rational Numbers

Consider the following numbers:

1. $$\frac{8}{3}$$

2. $$\frac{3}{0}$$

3. $$\frac{4}{-2}$$

4. $$\frac{5}{5}$$

5. $$\frac{ ext{5}}{ ext{x}}$$, where $$ ext{x}$$ is irrational.

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