<p>[Although}-A] the 500-mile course covers the [harshest}-B]
part of the Alaskan tundra, [scores of}-C] dogsled teams
[complete}-D] it successfully each winter since 1925.
[No error}-E]</p>
<p>answer is...
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.D</p>
<p>can you explain why?</p>
<p>The error here is in the tense usage. Since those team have completed “it” successfully each winter since 1925 (The action has started in the past but is still happening), it is supposed to read “have completed”.</p>
<p>This is my explanation though; correct me if I’m wrong.</p>
<p>Agree with Kitty.</p>
<p>thanks for explanation.</p>
<p>If the action has started in the past and still continues, shouldn’t it be “have been completing”?</p>
<p>very nice, very nice.</p>