Hello!
While there has been some great discussion about this issue, let's add our two cents!
Let's first figure out logically what the modifier and antecedent should be:
There are over
110 million dogs and cats in the United States,
which is more than the population of any Western European country.
Modifier = which is more than the population of any Western European country
Antecedent = 110 million dogs and cats
So what part of speech is "in the United States" then??The problem we're dealing with here is tricky.
The phrase "in the United States" is NOT the noun you're looking for - it's a prepositional phrase that has been placed between the noun and the "which" phrase. In most cases, it can muddle or mislead readers and you should not do this. However, in cases like this one, the reader can easily tell that the "which" phrase is referring to the number (110 million) and not the country.
Here are a couple examples of other sentences that do this without being confusing or misleading:
Horace made a donation to the new animal shelter in San Francisco, which was in desperate need of funds to get started.Modifier = which was in desperate need of funds to get started
Antecedent = animal shelter
Prepositional Phrase stuck between them = in San FranciscoPeople starting leaving during the first act of the film, which was before all of the cool stuff really started happening.Modifier = which was before all of the cools tuff really started happening
Antecedent = the first act
Prepositional Phrase stuck between them = of the filmRemember that the GMAT isn't asking you to find a
perfect answer - they want you to find the
best answer. In this case, option A isn't perfect - but it has far fewer (and less problematic) mistakes than the other options.
I hope this helps!