In the following question a sentence is divided into four parts. Find out which part of the sentence contains an error. If there is no error in the any part of the sentence, mark option 5 i.e. 'No error.' Ignore the error of punctuations, if any.

It's discourteous to (A)/ assume that we're so (B)/ dull that we would not (C)/ make these decisions ourselves.(D)/ No error (E)