Mark the letter A, B, C or D on...

Câu hỏi: Mark the letter A, B, C or D on your answer sheet to indicate the underlined part that needs correction in each on the following questions.
While working as a travel agency, my elder brother specialized in arranging tours of the Mekong Delta for foreign tourists.

A. a travel agency

B.  While

C.  specialized in

D.  tours of