A.\(\dfrac{{f\left( {{x_1}} \right)}}{{f\left( {{x_2}} \right)}} < 1\) với mọi \({x_1},{x_2} \in \mathbb{R};\,\,{x_1} < {x_2}\).
B. \(\dfrac{{f\left( {{x_2}} \right) - f\left( {{x_1}} \right)}}{{{x_2} - {x_1}}} > 0\) với mọi \({x_1},{x_2} \in \mathbb{R}\)
C.\(f\left( {{x_1}} \right) < f\left( {{x_2}} \right)\) với mọi \({x_1},{x_2} \in \mathbb{R};\,\,{x_1} < {x_2}\).
D.\(\dfrac{{f\left( {{x_2}} \right) - f\left( {{x_1}} \right)}}{{{x_2} - {x_1}}} < 0\) với mọi \({x_1},{x_2} \in \mathbb{R};\,\,{x_1} < {x_2}\)