Find the average rate of change for the given function.

$f(x)=x^2+3x$ between $x=0$ and $x=8$

What expression can be used to find the average rate of change of the function $f(x)$ over $[0,8]?$

A$.\frac{f(8)-f(0)}{8-0}$

B$.\underset{h0}{lim}\frac{f(0+h)-f(0)}{h}$

C$.\frac{f(8)+f(0)}{8+0}$

D$.\underset{x0}{lim}\frac{f(8)-f(0)}{8-0}$

The average rate of change of $f(x)$ over $0,8$ is $($Simplify your answer.$)$