If a continuous function $f(x)$ does not have a root in the interval $[a, b]$, then which one of the following statements is TRUE?
1. $f(a) . f(b)=0$
2. $f(a) . f(b) < 0$
3. $f(a) . f(b) > 0$
4. $f(a) / f(b) \leq 0$