Fill in the Blank with One of the Words or Phrases

Multiple Choice

Fill in the blank with one of the words or phrases listed below. Not all choices will be used. $$\begin{array} { l l l l } \text { least common denominator } & \text { simplifying } & \text { reciprocals } & \text { numerator } \\\text { cross products } & \text { ratio } & \text { proportion } & \text { denominator } \\\text { rational expression } & \text { domain } & \text { complex fraction } & \\\frac { - a } { b } & \frac { a } { - b } & \frac { - a } { - b } &\end{array}$$
-The _____of the rational function $f ( x ) = \frac { 1 } { x - 3 }$ is $\{ x \mid x$ is a real number, $x \neq 3 \}$ .

A) complex fraction
B) denominator
C) domain
D) numerator