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 of the following questions.
Though formally close friends, they have now been estranged from each other due to some regrettable misunderstandings.

A. formally

B. have now been estranged

C. each other

D. regrettable misunderstandings