21. The system of claim 18 , wherein the overall-relevance scores are generated according to the formulas: 
 ORS=W*RGA +(1 −W )* RS, wherein ORS is an overall-relevance score, W is a weighting factor, RGA is a revenue-generation amount, and RS is a relevance score.
18. The system of claim 1 , wherein the unit-ranking module is configured to calculate overall-relevance scores for the extracted-query units based on the revenue-generation amounts and the relevance scores.