The passage tells us that "by adolescence the percentage of boys with asthma is about the same as the percentage of girls with asthma."
We are looking for the answer choice that--taken with the passage--would allow us to conclude that the number of adolescent boys with asthma is approximately equal to the number of adolescent girls with asthma.
Answer choice (C) states: "there are approximately equal numbers of adolescent boys and adolescent girls in the population."
Let's say answer choice (C) is true. Then we know that there are approximately an equal number of adolescent boys and adolescent girls, e.g. let's say there are 100 boys and 99 girls.
We know from the passage that "by adolescence the percentage of boys with asthma is about the same as the percentage of girls with asthma," e.g. 40% of adolescent boys have asthma and 40% of adolescent girls have asthma.
Well, 40% of 100 is 40, and 40% of 99 is 39.6. Thus, if we take answer choice (C) to be true, we can conclude that the number of adolescent boys with asthma is approximately equal to the number of adolescent girls with asthma.
Hope that clears things up! Please let us know if you have any other questions.
