scientists didn't know much about lung cancer——.
A before long B until recently C long before D in the past few years
人气：271 ℃ 时间：2020-04-07 17:34:30
解答
C long before 很久以前
A before long 不久后,肯定错了.
B until recently 直到最近,是和现在有关,不能用过去时didn't
D in the past few years 最近几年,理由同上.
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