2. (2007 Mathematics Olympiad Collaborative Summer Camp Test Paper (I) Question) Given the sequence $\left\{a_{n}\right\}$ satisfies $a_{1}=$ $2000, a_{2}=2007, a_{n+2}=a_{n+1}-a_{n}\left(n \in \mathbf{N}^{*}\right)$, then $a_{2007}$ equals
A. 2007
B. -2007
C. 7
D. -7
2. C Hint: $a_{1}=2000, a_{2}=2007, a_{3}=7, a_{4}=-2000, a_{3}=-2007, a_{6}=-7, a_{7}=2000, a_{x}=2007$, from this we can deduce:
$a_{n-6}=a_{n},\left\{a_{n}\right\}$ is a sequence with a period of 6.
Therefore, $a_{3007}=a_{3}=7$.