1. A robot starts from the origin on a number line, moving in the positive direction of the number line, with a program of advancing 4 steps and then retreating 3 steps. Suppose the robot moves forward or backward 1 step per second, and each step is one unit length, $x_{n}$ represents the number corresponding to the robot's position on the number line at the $n$-th second (for example, $x_{4}=4$, $x_{5}=3, x_{7}=1$). Then the result of $x_{2007}-x_{2011}$ is ( ).
(A) Positive number
(B) Zero
(C) Negative number
(D) Any of the above is possible
-1. B.
It is known that the robot advances one unit length every $7 \mathrm{~s}$. From
$$
2007=7 \times 286+5,2011=7 \times 287+2 \text {, }
$$

we get
$$
\begin{array}{l}
x_{2007}=286 \times 1+4-1=289, \\
x_{2011}=287 \times 1+2=289 .
\end{array}
$$

Therefore, $x_{2007}-x_{2011}=0$.