[Erin]

Originally posted by Hippo

Thank you Erin for the great piece of information.
But does your example of a GMAT question mean that there could be grammatical mistakes in the sentence : New York, which is the largest city in the United States, serves as the center of COMMERCIAL for the country. Or is it just a typo?

[:o)] :o Ooops! It's a typo. I'll correct it.

Thanks for pointing this out!