[9급 국가직 영어 2013년 6번]

밑줄 친 부분 중 어법상 옳지 않은 것은?
Noise pollution ① is different from other forms of pollution in ② a number of ways. Noise is transient: once the pollution stops, the environment is free of it. This is not the case with air pollution, for example. We can measure the amount of chemicals ③ introduced into the air, ④ whereas is extremely difficult to monitor cumulative exposure to noise.
영어
정답 ④

출처: 인사혁신처 공개 기출(공공데이터포털, 이용허락 제한 없음)