Xét dấu biểu thức: \(f\left( x \right) = \frac{{{x^2} + 4x - 21}}{{{x^2} - 1}}\) ta có 

A. \(f\left( x \right) > 0\) khi \(x \in \left( { - 7; - 1} \right) \cup \left( {1;3} \right)\)

B. \(f\left( x \right) > 0\) khi \(x \in \left( { - \infty ; - 7} \right) \cup \left( { - 1;1} \right) \cup \left( {3; + \infty } \right)\)

C. \(f\left( x \right) < m 0\) khi \(x \in \left( { - \infty ; - 7} \right) \cup \left( { - 1;1} \right) \cup \left( {3; + \infty } \right)\)

D. \(f\left( x \right) < 0\) khi \(x \in \left( { - 7;0} \right) \cup \left( {1;3} \right)\)