EconomistGMATTutor wrote:Hello!

Thanks for your question. Option C isn't the best answer because it breaks up the phrases that describe mild flus too much. It creates a "non-essential" phrase that really is essential to the meaning of the sentence.

One way to make sure your commas are in the right place is to read the sentence through without each of the phrases that have commas on both sides. The sentence should still make perfect sense whether those phrases are there or not. In this case, if you took out the phrase "treatable with bed rest," you'd be left with this:

Varieties of influenza range in severity from mild flus, and they last only a few days, to acute forms, such as bacterial pneumonia, that are potentially fatal.

Since this doesn't really make sense without that phrase, we know the commas aren't in the right places. In answer A, if we took out the non-essential phrase, you are left with this:

Varieties of influenza range in severity from mild flus to acute forms, such as bacterial pneumonia, that are potentially fatal.

This still makes sense, even without that phrase added in. That means the commas were in the proper places, telling us that phrase could be taken out without changing the meaning of what you're saying.

I hope this helps! I'm available if you'd like any follow up.