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. Although Greek and Latin were

B. of extreme important during their day

C. they had become dead languages

D. by the beginning of the fifteenth century

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