January 2013 Writing Section -- Alan Turing Question?

Test Preparation SAT Preparation
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<p>January 2013 SAT 25 minute writing section – not experimental – identifying sentence errors:</p>

<p>In 1936, Alan Turing proved that a digital machine, which at the time exists only in theory, could perform the tasks of any other information-processing machine.</p>

<p>I put exists as the error. It shouldn’t be no error, because according to another thread there were 3 no error answers and this question wasn’t among them. According to the thread, the three No errors were:</p>

<p>Some person used the ideas of educators to create a children’s program
Some lawyer worked hard to “adequately preserve” the landmark
Some person used art forms from Africa or something</p>

<p>So is “exists” the error I was looking for?</p>

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<p>Yes. It should have been existed, because “at the time” refers to 1936, which requires past tense. Tricky question, because if you ignored the “at the time,” you might assume that said digital computer still doesn’t exist.</p>

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<p>…which at the time <em>existed</em> only in theory… is correct.</p>