1. A method for building a classifier, comprising: preparing, by a processor, a plurality of pairs for an information retrieval task, each of the plurality of pairs including (i) a training-stage speech recognition result for a respective sequence of training words and (ii) an answer label corresponding to the training-stage speech recognition result; obtaining, by the processor using a search engine, a respective rank for the answer label included in each of the plurality of pairs to obtain a set of ranks; determining, by the processor, for each of the plurality of pairs, an end of question part in the training-stage speech recognition result based on the set of ranks; building, by the processor, the classifier such that the classifier receives a recognition-stage speech recognition result and returns a corresponding end of question part for the recognition-stage speech recognition result, based on the end of question part determined for the plurality of pairs; splitting, by the processor, the recognition-stage speech recognition result at the corresponding end of question part for the recognition-stage speech recognition result; generating, by the processor, a set of answer candidates for replying to the corresponding end of question part for the recognition-stage speech recognition result; and providing, by a display device, the set of answer candidates to a user.