p366의 9번 문제인데
Q, the word 'they' in the passage refers to
a,miners
b,tunnels
c,galleries
d,columns
New problems arose with working far beneath the earth's surface, incuding a lack of proper ventilation for miners, and an increased likelihood of tunnels and mining galleries collapring, as thick supporting columns comprised of the hardest black coal were chiseled away because they were considered too valuable to remain untapped.
인데 답이 왜 d인지 잘모르겟습니다.