Noise pollution was a serious problem here .A. in the pastB. in the futureC. since thenD.

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(答案→)A. in the past
B. in the future
C. since then
D. once a week
解析：in the past在过去；in the future在将来；since then从那时以来；once a week每周一次。句意：过去在这儿噪音污染是一个严重的问题。结合本句谓语动词，可知应用表示过去时间的时间状语，故选A。