The figure below contains the graphs of four functions $f(x)$ , $f \left( \frac{1}{2} x \right)$ , $f(-2x)$ ,   and $f \left( -\frac{1}{2} x \right)$.

Identify which graph A-D is paired with each of the four functions by entering the correct letter in the answer box beside each expression.

(a) $f(x)$ is graph (enter a letter A-D)

(b) $f \left( \frac{1}{2} x \right)$ is graph (enter a letter A-D)

(c) $f(-2x)$ is graph (enter a letter A-D)

(d) $f \left( - \frac{1}{2} x \right)$ is graph (enter a letter A-D)