The Laplace transform $F(s)$ of the exponential function, $f(t) = e^{at}$ when $t \geq 0$, where $a$ is a constant and $(s-a) >0$, is

1. $\dfrac{1}{s+a} \\$
2. $\dfrac{1}{s-a} \\$
3. $\dfrac{1}{a-s} \\$
4. $\infty$