In each of these questions, a sentence has been divided into four parts and marked a. b, c and d. One of these parts contains a mistake in grammar Idiom or syntax. Identify that part and mark it as the answer.

A. On entering the meeting hall

B. loud cheers greeted the Prime Minister

C. who acknowledged them with a smile

D. and waved back happily at the gathering

Please do not use chat terms. Example: avoid using "grt" instead of "great".