Which of the following statements are correct when deciding whether to use the z or the t distribution?
选项：

A:Use the z distribution in a test of proportions when nπ and n(1 - π) are greater than or equal to 5.;
B:Use the z distribution when we have a normal population and we know the population standard deviation.;
C:Use t when the population is normal and the population standard deviation is not known.;
D:Use the t distribution in a test of proportions when nπ and n(1 - π) are greater than or equal to 5.