밑줄 친 (A),(B),(C)에서 문맥상 맞는 표현으로 가장 적절한 것… | [법원직 9급 영어] 13년 1회차 기출문제 상세 페이지

밑줄 친 (A),(B),(C)에서 문맥상 맞는 표현으로 가장 적절한 것은? 
It seems odd that a sound can sometimes be heard at its source and (A)vanish / appear at a distance, only to be heard again still farther away. For a long time this “zone of silence” defeated all attempts at its explanation. At length a meteorologist discovered that layers of air of different temperature interfere with sound. Since the air immediately above the ground is usually warmer than that in higher layers the sound waves are deflected diagonally upwards. The consequences are easily deduced. A certain distance from the source a sound becomes (B)inaudible / audible because it passes over our heads. At a height of about twenty-five miles, however, there is usually another layer of warm air and this deflects the sound back towards the earth. This explains why there is a region of audibility (C)within / beyond the zone of silence.

(A) (B) (C)

appear …… inaudible …… within

appear …… audible …… beyond

vanish …… inaudible …… beyond

vanish …… audible …… beyond