Let a and $b$ be positive integers, where a and $b$ do not equal $0.$ For what limit expression does L'Hospital's Rule apply? $(1$ point$)$

$\underset{x}{lim}\frac{1}{\sqrt[4]{x}}$

$\underset{xa}{lim}\frac{x+a}{x^2+a^2}$

$\underset{x0}{lim}\frac{x^9}{x^b}$

$\underset{x0}{lim}\frac{a^x}{x}$