Hi LulaBrazilia,

This prompt is an example of an Inference question. We need to understand the facts that are given, then pick an answer that logically matches those facts.

It's worth noting that an inference can be based on one word, a sentence or a combination of sentences, so we'll need to do a bit more work than normal to answer this question correctly.

The Facts:
-In 1987, the most common chronic medical conditions in the US were sinusitis, arthritis and high blood pressure (in that order).
-Arthritis and high blood pressure increase with age, but the incidence rate for sinusitis is the SAME for all ages.
-The AVERAGE AGE of the US population will INCREASE between 1987 and 2000

The Logic:
-Since the AVERAGE AGE of the US will INCREASE between 1987 and 2000, then the medical ailments listed will clearly be affected. Since people are living longer, the "numbers" behind each of these chronic medical conditions will be affected. Since arthritis and high blood pressure increase with age, it's likely that we'll see more people with those conditions (since people are living longer). Since sinusitis is consistent for all ages, we won't necessarily see an increase in sinusitis, but since people are living longer, that number will still be affected.

The correct answer C combines the Facts in the second and third sentences. Since people are living longer, and sinusitis occurs with the same frequency for all ages, the average age for sinusitis will most certainly increase.

GMAT assassins aren't born, they're made,
Rich