2. In the sequence $\left\{x_{n}\right\}$, $x_{1}=2, x_{2}=7, x_{n+2}$ is the unit digit of $x_{n} \cdot x_{n+1}$, $x_{1998}=(\quad)$.
A. 5
B. 2
C. 6
D. 8
2. B. After calculation, it is found that starting from the third term, the sequence has a period of 6, hence $x_{2000}=x_{8}=2$.