Bunuel
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.
A. asking users to blacklist the addresses sending them
B. asking users to blacklist the addresses, sending them
C. by asking users to blacklist the addresses from which they are sent
D. ask users to blacklist the addresses sending
E. by asking users to blacklist the addresses that send
OFFICIAL EXPLANATION
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.A. asking users to blacklist the addresses sending them
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.B. asking users to blacklist the addresses, sending them
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.C. by asking users to blacklist the addresses from which they are sent
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 verb: filtering. Or, however, is followed by a preposition: by.
What helps us identify this question as a Parallelism question as well as identify the mistake is the following Stop Sign: (n)either... (n)or...D. ask users to blacklist the addresses sending
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 verb. Or, however, is followed by ask, a base formverb.E. by asking users to blacklist the addresses that send
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 verb: filtering. Or, however, is followed by a preposition: by.