问题如下：
If a population is normally distributed with a known variance, which of the following statements on a hypothesis tests of the population mean is most accurate?
选项：
A. z-test.
B. t-test.
C. F-test.
解释：
A is correct, if the population is normally distributed with a known variance, then the test statistic for a hypothesis test concerning a single population mean, the Z-test is applied.
我被这题的答案弄糊涂了。总体方差已知，要么是x方，要么是F test。怎么是T test。T test不是已知mean吗？
