The spraying of pesticides can be carefully planned, but accidents,

You're right about which phrase is active and which is passive, but you should not conclude from this question that the GMAT "shows no preference" between active and passive phrasings. The active/passive distinction is not a grammatical one - passive constructions are just as "correct", grammatically, as active ones. But it's very often true that passive constructions are bad writing, and the GMAT shows a clear preference for good writing over bad. The most you should conclude from this one question is that there are situations where passive constructions are preferable to active ones, which is true (I just used a passive construction in this sentence!).

Writing tends to be easiest to read when it is clear who is performing the action. Both of these sentences are, technically speaking, grammatically correct:

The store was gone to by me.
I went to the store.

but the second active sentence is almost infinitely better than the first passive one (which is completely unacceptable English). There are situations though where passive constructions are justifiable. For example, you'd often prefer the first, passive construction below to the second:

Littering is banned in Canada.
The government banned littering in Canada.

The reason passive voice is perfectly acceptable here is because "littering" is presumably the subject, in the normal non-grammatical sense of that word, of the sentence - the likely point of the sentence is to point out the status of littering, not to point out who introduced certain legislation. Unless the sentence is trying to draw attention specifically to the actions of "the government", there's no reason to mention who banned littering at all.

So a decent guideline (one that probably has exceptions) is this: the subject of your sentence should be the subject of your verb. Most of the time, that means you should use active voice, because the person (or thing) performing the action in the sentence is usually who (or what) the sentence is primarily about. But there are exceptions, ones that do sometimes show up on the GMAT, where passive voice is better than active voice. And if the only distinction between two answer choices in SC is that one is active and one is passive, you'll be right the vast majority of the time if you pick the active one (though not 100% of the time).
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