At a school of 900 students, 20% have blue eyes. A student randomly selects 100 students and finds 17% of them have blue eyes. A second student takes another random sample of 90 students and finds 24% of them have blue eyes. Which of the following explains why there is a difference between the two percentages? a. The sample sizes were both too small.
b. The samples were not random samples.
c. Both samples suffered from non-response bias.
d. Random error; the numbers were different due to variability inherent in sampling.