An ellipse is given in standard form by $\frac{x^2}{25}+\frac{y^2}{9}=1$. Which lines are the directrices of the ellipse?

• A. $x=\pm \frac{5}{4}$
• B. $x=\pm \frac{25}{4}$
• C. $y=\pm \frac{9}{4}$
• D. $x=\pm \frac{25}{3}$

Answer: (B)