Evaluate $\underset{x}{lim}f(x)$ and $\underset{x-}{lim}f(x)$ for the rational function. Then give the horizontal asymptote of $f($if any$).$

$f(x)=\frac{21x^6-4}{3x^6-8x^5}$

Select the correct choice below, and, if necessary, fill in the answer box to complete your choice.

A$.\underset{x-}{lim}\frac{21x^6-4}{3x^6-8x^5}=($Simplify your answer.$)$

B$.$ The limit does not exist and is neither $$ or $-.$