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Given $\mathop {\lim }\limits_{x \to {3^ - }} f(x) = \infty$. Which of the following must be true?

I. $f(x)$ has a horizontal asymptote at $y=3$II. $f(x)$ has a vertical asymptote at $x=3$III. $f(x)$ is unbounded

I only

II only

I and II only

II and III only