1. A computer program product comprising a computer readable storage medium having a computer readable program stored therein, wherein the computer readable program, when executed on a computing device, causes the computing device to: receive an input question for generation of an answer to the input question; generate a set of candidate answers for the input question based on an analysis of a corpus of information, wherein each candidate answer in the set of candidate answers corresponds to an evidence passage supporting the candidate answer as answering the input question; determine, based on the set of candidate answers, whether clarification of the input question is required; identify, in response to a determination that clarification of the input question is required, a differentiating factor in evidence passages of at least two candidate answers in the set of candidate answers; send, by the data processing system, in response to a determination that clarification of the input question is required, a request for user input to clarify the input question, wherein the request for user input is generated based on the identified differentiating factor; receive user input from the computing device in response to the request; and select at least one candidate answer in the set of candidate answers as an answer for the input question based on the user input, wherein identifying a differentiating factor in evidence passages of at least two candidate answers in the set of candidate answers comprises: identifying a plurality of differentiating factors between evidence passages of the at least two candidate answers; and selecting a subset of differentiating factors from the plurality of differentiating factors based on an evaluation of which differentiating factors in the plurality of differentiating factors clarify the input question.