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Sentence Correction: Parallelism - (n)either A (n)or B

Anti-spam software can prevent unwanted mail by either filtering incoming messages with the aid of rule-based filters or asking users to blacklist the addresses sending them junk mail.

Well done!

The original sentence correctly uses parallelism with the either...or structure: by either (1) filtering ... or (2) asking ...

Although this answer choice is stylistically flawed because of a pronoun ambiguity (them can refer either to addresses or users), it is the only answer choice that is grammatically correct and retains the original meaning. Therefore, it is the best answer choice.

Incorrect.

This answer choice is grammatically incorrect. In the list by either filtering ... or ..., the part of speech that follows either must also follow or. In this answer choice, either is followed by a verbfiltering. Or, however, is followed by a preposition: by

[[snippet]]

Incorrect.

This answer choice is grammatically incorrect. In the list by either filtering ... or ..., the part of speech that followseither must also follow or. In this answer choice, either is followed by a verbfilteringOr, however, is followed by apreposition: by

[[snippet]]

Incorrect.

This answer choice changes the original meaning of the sentence. By adding a comma, it separates the phrase sending them junk mail from the original the addresses (that are) sending them junk mail. This new separate phrase implies that the anti-spam software sends junk mail.

Incorrect.

This answer choice is grammatically incorrect. In the list by either filtering ... or ..., the part of speech that follows either must also follow or. In this answer choice, either is followed by filtering, a V+ing verbOr, however, is followed by ask, a base formverb.

[[snippet]]
asking users to blacklist the addresses sending them
asking users to blacklist the addresses, sending them
by asking users to blacklist the addresses from which they are sent
ask users to blacklist the addresses sending
by asking users to blacklist the addresses that send