The GMAT has not put down clear rules about the use of the two voices. However, the overall understanding of the test indicates that on the test the active voice is preferred to the passive voice provided there are no structural limitations of the sentence that compel otherwise.
The lights of the aurora borealis shift up north at dusk by the movement of the earth on its axis.
As the sentence stands, the underlined portion is in active voice; the meaning conveyed is that the lights shift themselves; however, the non-underlined portion of the sentence includes a phrase “by the movement of the earth on its axis.” This phrase clearly indicates that the lights “are shifted” by the movement of the earth and so the voice of the sentence needs to be in passive form. In this example we will be obliged to use the passive voice because of the structural limitations of the sentence. If the “by the movement………….” part of the sentence was not included or if it was underlined and hence not obligatory to abide by, we would have taken active voice because in the absence of information to prove otherwise we will presume that the lights moved on their own.
Parallelism is applicable even for the Voice of a sentence. This might not have been taught as a part of ‘school’ grammar but then a lot else that the GMAT throws up has also never been taught! Or that is what GMAT test takers normally say in defense of ignorance. Anyway here is a simple but telling example of how one can tackle the voice issues in a sentence by focusing on parallelism.
In December of 1987 an automobile manufacturer pleaded no contest to criminal charges of odometer tampering and agreed to pay more than $16 million in civil damages for cars that were test-driven with their odometers disconnected.
(A) cars that were test-driven with their odometers disconnected
(B) cars that it had test-driven with their disconnected odometers
(C) its cars having been test-driven with disconnected odometers
(D) having test-driven cars with their odometers disconnected
(E) having cars that were test-driven with disconnected odometers
In the non-underlined portion of this sentence you can spot a clear parallel of two verbs ‘pleaded’ and ‘agreed’ that both are actions done by the subject ( an automobile manufacturer) and are hence evidently in Active Voice. Yet, after the sentence enters the underlined portion we suddenly encounter ‘cars that were test driven’ – a phrase that suggests that the cars were driven by someone but not essentially the automobile manufacturer and this gives us a sense of disconnect with the previous two verbs. Moreover, the logical interpretation of the sentence asks why the manufacturer would plead guilty and agree to pay a fine for cars that were test driven by someone else! We definitely need the manufacturer to be the one who had driven the cars and so we push for an Active Voice sentence. This limits us to (D) and (E). A closer reading of (E) allows us to discover the same Passive Voice tone that rendered (A) (B) and (C) inappropriate. So, it is (D).
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