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我也选成了E,所以google了一下,以下是manhattan中的大牛Ron对E的讲解: inappropriate tense. if the past perfect is used to describe a state or description of something (as opposed to an action verb), it should generally be used to describe a state/description that is no longer the case. since leaching is presumably still an extraction method (this is not the sort of thing that is subject to change), the past perfect is inappropriate.
also, the modifier (starting with "well established") shouldn't be a nonessential modifier, i.e., it shouldn't be set off by commas. this isn't generally a difference that's tested, so i'll defer the explanation to the following thread, on which i wrote about it: post45536.html#p45536 after you read that part, you should better understand why the nonessential modifier doesn't work here. |
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