11) What is the unit digit of $1^{2}+2^{2}+3^{2}+\ldots+1996^{2}$?
(C) 4
(D) 6
(E) 8.
11) The answer is (D).

Indeed, the unit digit of $1^{2}+2^{2}+3^{2}+\ldots+10^{2}$ is 5 (direct verification), in the same way, it can be seen that $11^{2}+12^{2}+$ $\cdots+20^{2}=(10+1)^{2}+(10+2)^{2}+\cdots+(10+10)^{2}=10 \cdot k \cdot\left(1^{2}+2^{2}+\cdots+10^{2}\right)$ has 5 as the unit digit. The same applies to $21^{2}+22^{2}+\cdots+30^{2}$, and so on. Furthermore, the unit digit of $1991^{2}+\ldots+1996^{2}$ is equal to 1. Therefore, the result is equal to the last digit of $199 \cdot 5+1$, which is 6.