首先说the last few decades和 the past few decades是一样的，所以问题就变成in 和over在过去一段时间上的区别问题。 两者都可以翻译成在过去的几十年中/内， 用in表示在. In a decade = in 10 years time this has no specific date. In the/this decade = in these 10 years this implies you're talking about the current decade.
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I mean we can say two decades or three decades to.
