안녕하세요 ~

오늘 문제를 풀다가 좀 궁금한게있어서요.

the united states coast guard maintains lightships, anchored at dangerous places along the coast and at the entrances to certain ports. these lightships are equipped with two masts similar to those found in lighthouses.
one searchlight burns every day from sunset to sunrise, and the other is kept in reserve.

정답은 on which are mounted powerful searchlights 인데요 보기중에 which powerful searchlights are mounted on
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좀알려주세요~!!