I received a PM asking me to comment on usage of have/has vs. have been/has been here.
In this case, I'm guessing that the question is really about the distinction been has shifted and has been shifted.
Both of these are present perfect, which is constructed by using "have" or "has" plus the past participle (regular form ends in -ed). "Has BEEN shifted" is also passive because "been" is a form of the verb "to be."
So the question here is: should this sentence be active or passive? As a general rule, we prefer active to passive, BUT when we're dealing with some things set in stone (that is, the non-underlined stuff), we have to look at the whole sentence set-up to see if it dictates active or passive.
So, strip it down:
Light (has shifted / has been shifted) toward X by Y.
Or even more simply:
Light (has shifted / has been shifted) by Y.
Is the light shifting itself? Or is the light being shifted by something else? If the former, use active. If the latter, use passive. In this case, it's the latter, so we need the passive "has been shifted." A is the right answer.
When you're deciding between active and passive, the inclusion of the preposition "by" often indicates a passive sentence. The noun following the "by" is the thing that's actually performing the action indicated by the verb. And if the noun performing the action is within a prepositional phrase, then that noun is not the subject... so the subject is not performing the verb (action). If that's the case, the subject is having that action performed on it... and that dictates passive structure.
You can also try what I did above: ask yourself whether the subject (light) is performing the action (shifting itself) or whether the subject (light) is having the action performed on it (being shifted by something else). This can help clarify whether you need active or passive.
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Stacey Koprince
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Manhattan GMAT
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