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In the following 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. Each of the speakers whom I invited

B. to participate in the debate

C. has indicated their unwillingness

D. because of the short notice

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