<p>Tornadoes form when large air masses of different temperatures collide, which results from either a change in the direction or the speed of the wind.</p>
<p>(A) collide, which results from either a change in<a href="B">/u</a> collide, resulting either from a change in
(C) collide as the result of a change in either</p>
<p>As IceQube has said, A has a modifier problem. However, B does not. The problem with B lies in the lack of parallelism(although this is not the only problem with choice B)</p>
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<p>This must be changed to:</p>
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<p>Since B lacks parallelism, it is not a viable choice for the answer.</p>
<p>C is correct because C fixes the problem of parallelism evident in B.</p>
<p>On a side note, participial phrases can be attached anywhere in the sentence – though some places are preferable to others. They modify the subject of the sentence.</p>
<p>I’m not sure what limabeans means with the noun and the verb, but the way he/she corrected the sentence is incorrect, in my opinion.</p>
<p>I think part of the sentence after “collide” is used to explain why the collisions occur. If the sentence is corrected the way limabeans did, the meaning of the sentence is changed. This problem also exists in choice B(this was the problem that I mentioned earlier).</p>
<p>There are [rare] exceptions to the parallelism rule.
Consider
When<a href=“A”>/u</a> L’Enfant suggested to build<a href=“B”>/u</a> the United States Capitol at the center of<a href=“C”>/u</a> Washington, D. C., he also offered<a href=“D”>/u</a> to design a landscaped mall nearby. (E)</p>
<p>Nice parallel structure, right?
Well, the answer is “B”: the verb “suggest” should be followed by a gerund, not an infinitive.
When L’Enfant suggested building the United States Capitol at the center of Washington D.C., he also offered to design a landscaped mall nearby.
No parallelism no more. :)</p>
<p>As far as parallelism, after correcting the sentence
the original parallel
“suggested to build” and “offered to design”
become
“suggested building” and “offered to design”.</p>