Read the 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 correct as it is, then choose option ‘No Error’ as your answer response. (Ignore errors of punctuation, if any).

I was unable to understand (A)/ how the plan was going to work (B)/ unless the group leader created (C)/ an easy-to-follow diagram on the board (D).