Question on Comparison

[bpd]

This question is from Foundations of GMAT Verbal, 5th Edition.
This question is the first question in Drill 6.2 on page 106 of the aforementioned volume.

Correct/Incorrect: New York City's population is greater than Montana, Wyoming, Vermont, North Dakota, Delaware, and Rhode Island all added together.

Incorrect. Corrected version: New York City's population is greater than those of Montana, Wyoming, Vermont, North Dakota, Delaware, and Rhode Island all added together.

My doubt
Can the correct answer also be "New York City's population is greater than that of Montana, Wyoming, Vermont, North Dakota, Delaware, and Rhode Island all added together."

If the answer is no, then could you explain why this would not be correct?

Thanks.

[jlucero]

That would be incorrect. Those = plural. That= singular. Since the sentence ends with "all added together" we need to have populationS added together, not one population.