Mark the letter A, B, C, or D to show the $\underline{underlined}$ part that need correction in each of the following sentences.

Câu 9. Today the (A) $\underline{number}$ of people (B) $\underline{whom}$ enjoy winter sports (C) $\underline{is}$ almost double (D) $\underline{that}$ of twenty years ago.