Let the mean and standard deviation of marks of class A of 100 students be respectively 40 and α(>0) , and the mean and standard deviation of marks of class B of n students be respectively 55 and 30 -α. If the mean and variance of the marks of the combined class of 100 + n students are respectively 50 and 350, then the sum of variances of classes A and B is
(1) 500
(2) 650
(3) 450
(4) 900
