Can someone please help me to understand this? A simple random sample of 1200 adult Americans is selected, and each person is asked the following question. "In light of the huge national deficit, should the government at this time spend additional money to establish a national system of health insurance?" Only 39% of those responding answered "yes."

This survey
A) is reasonably accurate since it used a large, simple random sample.
B) probably overstates the percentage of people that favor a system of national health
insurance.
C) probably understates the percentage of people that favor a system of national health
insurance.
D) is very inaccurate, but neither understates nor overstates the percentage of people
that favor a system of national health insurance. Since simple random sampling
was used, it is unbiased.

I know the answer is "C" but I genuinely don't understand why it is. If someone could explain it'd be much appreciated, thanks.