57.

Answer and Explanation

Your Answer is

Correct Answer is A

Explanation

Item B/C: rapid and tapping are two adjectives, which are modifications to sound. The two adjectives are adjectives of different levels. There is no comma in between, B/C is wrong.

Item D: they make is an attributive clause of sound, and that is omitted, so there cannot be a comma before they, and D is wrong.