An independent t-test is used to test for:
a. Differences between means of groups containing different people when the data are normally distributed, have equal variances and data are at least interval.
b. Differences between means of groups containing different people when the data are not normally distributed or have unequal variances.
c. Differences between means of groups containing the same people when the data are normally distributed, have equal variances and data are at least interval.
d. Differences between means of groups containing different people when the data are not normally distributed or have unequal variances.