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Sentence Correction: Tenses - Verb Forms

The distinction between a celestial object known as a planet with one defined as a star is only valid insofar as its mass is lower than a certain threshold, over which it can become subject to nuclear fusion.

Incorrect.

This answer choice is grammatically incorrect. Using the preposition with in the phrase distinction between [A] with [B] is unidiomatic. The correct idiom is distinction between [A] and [B].

In addition, this answer choice is stylistically flawed. The pronoun its is ambiguous because it can refer to more than one singular noun in the previous part of the sentence (object, planet, star).

[[snippet]]

Incorrect.

While this answer choice corrects the original Pronoun Ambiguity mistake, it repeats the idiom mistake in the original question. Using the preposition with in the phrase distinction between [A] with [B] is unidiomatic. The correct idiom is distinction between [A] and [B].

In addition, by changing the singular verb is to the plural verb are this answer choice creates a Subject Verb Agreement mistake. Although mass ends with an s, it is a singular noun.

Incorrect.

While this answer choice corrects the Pronoun Ambiguity mistake in the original question, it is grammatically incorrect. The use of the base form be instead of the original is leaves the clause without a conjugated verb.

In addition, using the preposition of in the phrase distinction of [A] and [B] is unidiomatic. The correct idiom is distinction between [A] and [B].

Incorrect.

This answer choice is grammatically incorrect. While this answer choice attempts to corrects the Pronoun ambiguity mistake in the original question, by changing its mass to their mass, it creates a Pronoun Agreement mistake. Their can only refer to a plural noun, but there is no plural noun in the preceding part of the sentence. 

[[snippet]]

In addition, using the phrase distinction [A] has from [B] is unidiomatic. The correct idiom is distinction between [A] and [B].

Well done!

This answer choice corrects the original Idiom mistake, by changing with to and. The correct idiom is distinction between A and B. It also corrects the Pronoun Ambiguity mistake in the original question, by changing its mass to the object's mass.

[[snippet]]

Note that the use of one in the phrase one defined as a star is a correct idiomatic usage. Although one refers to a celestial object, it is not a pronoun of the same type as one that refers to humans (e.g. One must love one's children).

between a celestial object known as a planet with one defined as a star is only valid insofar as its mass is
between a celestial object known as a planet with one defined as a star is only valid insofar as the object's mass are
of an object known as a planet and one defined as a star is only valid insofar as the object's mass be
between a celestial object known as a planet and one defined as a star is only valid insofar as the object's mass is
a celestial object known as a planet has from one defined as a star is only valid insofar as their mass is