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. $$\begin{array} { l l l l } \text { least common denominator } & \text { simplifying } & \text { reciprocals } & \text { numerator } \\\text { cross products } & \text { ratio } & \text { proportion } & \text { denominator } \\\text { rational expression } & \text { domain } & \text { complex fraction } & \\\frac { - a } { b } & \frac { a } { - b } & \frac { - a } { - b } &\end{array}$$
-If $\frac { \mathrm { a } } { \mathrm { b } } = \frac { \mathrm { c } } { \mathrm { d } }$ , then ad and bc are called______

A) least common denominator
B) cross products
C) complex fraction
D) reciprocals