Sentence Correction: Subject–Verb Agreement: A Pseudo-Plural Singular Subject

Whooper swans were sighted yesterday evening flying over the island of Inch; it seems the white flock arrived from the north and are heading southwestwards.

Incorrect.

This answer choice is grammatically incorrect. The singular subject flock does not agree with the plural verb are.

[[snippet]]

Flock is a confusing word. It refers to more than one animal, which makes us think of it a plural subject, but it is, in fact, singular (the plural is flocks).

Incorrect.

This answer choice repeats the original mistake. The singular subject flock does not agree with the plural verb are.

[[snippet]]

Flock is a confusing word. It refers to more than one animal, which makes us think of it a plural subject, but it is, in fact, singular (the plural is flocks).

Incorrect.

While this answer choice corrects the original mistake, by changing the plural verb are to the singular verb is, it creates a new Subject Verb Agreement mistake. The singular subject flock does not agree with the plural verb have.

[[snippet]]

Flock is a confusing word. It refers to more than one animal, which makes us think of it a plural subject, but it is, in fact, singular (the plural is flocks).

Great!

This answer choice corrects the original Subject Verb Agreement mistake by changing the plural verb are (heading) to the singular verb is (headed), to match the singular subject flock.

Note that the difference between heading and headed is insignificant in this case, and does not affect the choice of the correct answer. The two phrases mean the same thing. 

In addition, although this answer choice is more wordy than the others, it is the only answer choice that is grammatically correct. According to the Sentence Correction Work Order, we only eliminate wordy answer choices if we have eliminated all the answer choices with grammatical/logical/redundancy mistakes and still are left with more than one remaining answer to choose from.

Incorrect.

This answer choice replaces the original mistake with a similar Subject Verb Agreement mistake. The plural subject swans does not agree with the singular verb is.

Remember: in Subject Verb Agreement questions, if both the subject and the verb are underlined, you need to check them both. You cannot assume that the subject will remain the same in all answer choices and that only the verb will change, and vice versa: you cannot assume that the verb will remain the same in all answer choices and that  only the subject will change.

flock arrived from the north and are heading southwestwards

flock have arrived from the north and is heading southwest

flock arrived from the north and is headed in a southwesterly direction

swans arrived from the north and is heading southwest

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