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Sentence Correction: Pronoun Questions - Ambiguity

The 233-mile long aqueduct designed to deliver water from East of the Sierra Nevada to the expanding city of Los Angeles was the brain child of the visionary engineer William Mulholland, who believed that unless he gained control of all water rights in the Owens Valley, its supply was always insufficient.

Incorrect.

This answer choice is grammatically incorrect. The result part of the conditional in this sentence (was insufficient) is in the Past Simple tense, but this tense cannot be used in the result part of any of the three conditionals.

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In addition, the phrase its supply is ambiguous as the pronoun it could logically refer to more than one singular noun in the previous part of the sentence (Owens Valley, Los Angeles, aqueduct).

Incorrect.

While this answer choice corrects the Conditionals mistake in the original question, it is stylistically flawed. The phrase control for is non-idiomatic: the correct idiom is control of

In addition, the phrase be in sufficient supply of is wordy and awkward.

Incorrect.

Since this sentence is a conditional, its condition and result parts must be consistent (Conditional 1, 2, or 3).

The condition part of this conditional sentence is in the Past Simple tense (he did not gain control), indicating Conditional 2. However, the result part is in the Future Past Perfect (would have been), indicating Conditional 3.

[[snippet]]

In addition, the phrase its supply is ambiguous as the pronoun it could logically refer to more than one singular noun in the previous part of the sentence (Los Angeles, aqueduct).

Incorrect.

While this answer choice corrects the original Pronoun ambiguity mistake, it repeats the Conditionals mistake.The result part of the conditional in this sentence (was insufficient) is in the Past Simple tense, but this tense cannot be used in the result part of any of the three conditionals.

[[snippet]]

Excellent.

This answer choice corrects the original Pronoun Ambiguity mistake by changing its supply to the city's water supply.  It also corrects the original Conditionals mistake, by using the Future Past would never be sufficient to express the hypothetical nature of Mullholland's belief.

Note that it does not matter that this answer choice does not use the explicit unless/if formula to indicate the conditional, and replaces unless he gained control of by without control of. The conditional meaning is still preserved by the specific use of the Future Past tense.

unless he gained control of all water rights in the Owens Valley, its supply was always insufficient
if he did not gain control for all water rights in the Owens Valley, the city would never be in sufficient supply of water
the city's water supply would never be sufficient without control of all water rights in the Owens Valley
never would its water supply have been sufficient, if he did not gain control of all water rights in the Owens Valley
if he weren't in control of all the water rights in the Owen Valley, the city's water supply was not ever sufficient