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Sentence Correction: Subject Verb Agreement - X of Y Subject

Birds of a feather flocks together.

Incorrect.

This answer choice is grammatically incorrect. The subject is plural and the verb is singular

[[snippet]]

The subject is Birds of a feather. In the case of an X of Y subject, the verb should agree with X. Thus, the core subject is Birds, which is plural. 

Incorrect.

This answer choice is grammatically incorrect. The subject is singular and the verb is plural

[[snippet]]

The subject is A bird of a feather. In the case of an X of Y subject, the verb should agree with X. Thus, the core subject is A bird, which is singular.

This answer choice is grammatically incorrect. The subject is singular and the verb is plural

[[snippet]]

The subject is Bird of a feather. In the case of an X of Y subject, the verb should agree with X. Thus, the core subject is Bird, which is singular.

Well done!

This answer choice corrects the original Subject Verb Agreement mistake, by changing the singular verb flocks to the plural verb flock, to match the plural subject Birds

Incorrect.

While this answer choice corrects the original mistake, it creates another mistake by changing feather to feathers. The word a can only come before singular nouns, so a feathers is grammatically incorrect.

Birds of a feather flocks together
A bird of a feather flock together
Bird of a feather flock together
Birds of a feather flock together
Birds of a feathers flock together