Consider a population consisting of four children with ages, in years, of _, _, _, and _. The population mean age is _ years, and the population variance is _ square years.
a. 5, 7, 9, 11; 8; 4
b. 4, 6, 8, 10; 7; 6
c. 5, 6, 7, 8; 6.5; 3.5
d. 6, 7, 8, 9; 7.5; 4.25