The main difference between a $\chi^{2}$ test for independence and a~$\chi^{2}$ test for homogeneity of proportions is which of the following? 
选项为:A:They are based on a different number of degrees of freedom.  B:One of the tests is for a two-sided alternative and the other is for a one-sided alternative.  C:In one case, two variables are compared within a single population. In the other case, two populations are compared in terms of a single variable.  D:For a given value of $\chi^{2}$, they have different P-values.  E:There are no differences between the tests. They measure exactly the same thing.  请回答其中一个选项.