Let $f ( x ) = \frac { 2 x } { \sqrt { x ^ { 2 } + 1 } }$

Multiple Choice

Let $f ( x ) = \frac { 2 x } { \sqrt { x ^ { 2 } + 1 } }$ The set of all horizontal asymptotes of ƒ is

A) $\{ y = 2 \}$
B) $\{ y = - 2 \}$
C) $\{ y = 2 , y = - 2 \}$
D) $\{ y = - 1 , y = 1 \}$
E) $\varnothing$