A survey in 2007 of 447 parents nationwide by the nonprofit organization College Saving Foundation found that 27% had saved nothing for their children's future college expenses. Based on the data, a 95% confidence interval is (0.23, 0.31). How should we interpret this interval?
a. The probability is 0.95 that the sample proportion falls in this interval.
b. The probability is 0.95 that the population proportion falls in this interval.
c. We are 95% confident that the sample proportion falls in this interval.
d. We are 95% confident that the population proportion falls in this interval.