Read each sentence to find out whether there is any grammatical or idiomatic error in it. The error, if any, will be in one part of the sentence. The corresponding letter of that part is the answer. If the given sentence is grammatically and contextually correct, then choose “No error” as the correct answer. (Ignore errors of punctuation, if any.)

The teacher asked the students (A) / if they had completed (B) / their homework before (C) / coming to the class. (D)