1. If the sequence $\left\{a_{n}\right\}$ is defined as $a_{1}=2, a_{n+1}=a_{n}+2 n(n \geqslant 1)$, then $a_{100}$ equals
A. 9900
B. 9902
C. 9904
D. 10100
1. B From the known recurrence relation, we get $a_{2}-a_{1}=2 \cdot 1$,
$$
\begin{array}{l}
a_{3}-a_{2}=2 \cdot 2, \\
\cdots \cdots \cdots \cdots \cdots \cdots \\
a_{100}-a_{99}=2 \cdot 99 .
\end{array}
$$

By adding up both sides of these equations, we have $a_{100}-a_{1}=2(1+2+\cdots+99)=99 \cdot 100=9900$. Therefore, $a_{100}=9902$.
So the answer is B.