Solve the spotting error for the given direction

Read each sentence to find out whether there is any grammatical error in it. The error, if any will be in one part of the sentence. The letter of that part is the answer. If there is no error, the answer is 'D'. (Ignore the errors of punctuation, if any). (solve as per the direction given above) A. We discussed about the problem so thoroughly B. on the eve of the examination C. that I found it very easy to work it out. D. No error.