单选题 In a test of the mean of a population, if the population variance is:

A. known, a z- distributed test statistic is appropriate.
B. known, a t- distributed test statistic is appropriate.
C. unknown, a z -distributed test statistic is appropriate.

【正确答案】 A

【答案解析】If the population sampled has a known variance, the z - test is the correct test to use. In general, a t - test is used to test the mean of a population when the population variance is unknown. Note that in special cases when the sample is extremely large, the z - test may be used in place of the t- test, but the t -test is considered to be the test of choice when the population variance is unknown.