Fill in the Blank with One of the Words or Phrases

Fill in the blank with one of the words or phrases listed below. Not all choices will be used. $$\begin{array} { l l l l } \text { least common denominator } & \text { simplifying } & \text { reciprocals } & \text { numerator } \\\text { cross products } & \text { ratio } & \text { proportion } & \text { denominator } \\\text { rational expression } & \text { domain } & \text { complex fraction } & \\\frac { - a } { b } & \frac { a } { - b } & \frac { - a } { - b } &\end{array}$$
-A(n) ----------is an expression that can be written in the form $\frac { \mathrm { P } } { \mathrm { Q } } \text {, where } \mathrm { P } \text { and } \mathrm { Q } \text { are }$ polynomials and Q is not 0.

A) ratio
B) rational expression
C) proportion
D) complex fraction