Which Statement Is Correct Concerning the Decomposition of the Total

Which statement is correct concerning the decomposition of the total variation?

A) Because it is not known that all the groups have the same mean,we cannot calculate the variance of all the observations together.
B) If the population mean is the same in all the groups,then the variation in the sample means and the sizes of the sample groups can be used to estimate the variance of Y.
C) By comparing the Y variance estimates based on between-group and within group variation,we can test the null hypothesis.
D) All statements are correct.