Let the mean and standard deviation of marks of class A of 100 students be respectively 40 and $\alpha\ (> 0)$, and the mean and standard deviation of marks of class $B$ of $n$ students be respectively 55 and $30 - \alpha$. 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) 450\\
(3) 650\\
(4) 900