SAT Grammar question

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1

<p>I found this grammar particularly troubling since it goes against what I know about when to use the past participle:
Although still not visible from space as (A)is sometimes(/A) claimed, China's great wall, the nation's (B)most famous(/B) relic of ancient times, is significantly (C)longer than (/C) experts (D)had previously(/D) estimated.(E)No Error(/E). </p>

<p>The answer was E but I picked D. I thought had previously shouldn't work here because the past participle only described something that happened before in a past tense, but the tense of the sentence here seems to be present (...is sometimes... is significantly longer). Can anyone tell me why D is correct?</p>

2

<p>My grammar’s not perfect, but I don’t think “had previously” qualifies a tense change. After all, you wouldn’t want to read</p>

<p>"…longer than experts are previously predicting,"</p>

<p>would you?</p>

3

<p>Strip the sentence to its essence:</p>

<p>China’s great wall is longer than experts had previously estimated.</p>

<p>“had estimated” is the past perfect of the verb estimate.</p>

<p>The past perfect serves to talk about something that happened before a point in the past. “It looks back from a point in the past to further in the past.” The adverb “previously” emphasizes the two events – far past and past.</p>