19世纪以前，几乎没有社会流动的渠道。
Prior to the nineteenth century, there were almost no channels of social mobility.
一个世纪以前，石油资源是一大问题。
一个世纪以前，很少有人一天刷几次牙。
A century ago, few people regularly brushed their teeth multiple times a day.
然而，第一个慢性中毒病例发生在半个世纪以前。
However, the first chronic case took place half a century ago.
故事发生在大约半个世纪以前。
一个世纪以前，女性几乎没有投票权，更别说参选了。
CENTURY ago, women were scarcely allowed to vote, let alone stand for election.
17世纪以前，撇号很少被运用。
答案是近4个世纪以前——1638年12月21日。
The answer is nearly four centuries ago - December 21, 1638.
一个多世纪以前。
在印度，仅仅上个世纪以前，十个儿童一般要死九个。
In India, just before this century, nine children used to die out of ten.
然而，莱茵石已经有好几个世纪以前，最初切割和完成的手。
Rhinestones, however, have been around for centuries and were originally cut and finished by hand.
他们都是几世纪以前的人呢！
他们在一两个世纪以前所列出的因素中，大多数至今仍是很重要的。
Most of the factors they listed one or two centuries ago are still important today.
一个世纪以前美国西部还是一片边疆。
公元九、十世纪以前还是我国粮食生产的主要基地。
Huangfan Plain was the main grain-producing area ten century ago.
在此以前的几个世纪里。
这个城市建立于很多个世纪以前。
半个世纪以前很难想象去月亮上旅行。
Half a century ago it's difficult to conceive of traveling to the moon.
在我们进入一个新的世纪，很不幸地，这句话会比以前更适用。
As we move into the next century, this phrase is unfortunately more applicable than ever.
一个世纪以前美国西部还是一片边疆。
它们是1431年以前经过六个世纪才建成的。
They had been built over a period of six centuries up till 1431.
现在人们的睡眠时间相比一个世纪以前减少了20%。
People now sleep about 20% less than they did a century ago.
而若干世纪以前，尼罗河水泛滥，冲垮了古都。
C... But the Nile burst its Banks a few centuries ago, flooding the ancient capital.
一个多世纪以前就已经发现孕妇外周血中存在胎儿细胞。
Fetal cells were identified in maternal blood a century ago.
一个多世纪以前就已经发现孕妇外周血中存在胎儿细胞。
Fetal cells were identified in maternal blood a century ago.
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