hi, im having a problem with this question. Im not sure how even go about getting one of these multiple choices.

The graph of $\displaystyle f(x) = \frac{1}{x^2-mx -n}$, where m and n are real constants, has no vertical asymptotes if

A: $\displaystyle m^2 < 4n$

B: $\displaystyle m^2 > 4n$

C: $\displaystyle m^2 = -4n$

D: $\displaystyle m^2 < -4n$

E: $\displaystyle m^2 > -4n$

help appreciated.