Suppose a survey is taken of two groups of people where each person is asked a yes/no question and the proportion of people who answer yes is calculated for each group. Which of the following is true about a hypothesis test of the difference in the two proportions?

a) The t-distribution should be used if the standard deviations are unknown.

b) The standard deviation must be assumed equal.

c) Normality can be assumed if the sample size for each population is at least 30.

d) Normality can be assumed if, in each group, at least 5 people say yes, and at 5 people say no.