6. Let $f(x)$ be a function defined on $\mathbf{R}$. If there exist two distinct real numbers $x_{1}, x_{2} \in \mathbf{R}$, such that $f\left(\frac{x_{1}+x_{2}}{2}\right)=\frac{f\left(x_{1}\right)+f\left(x_{2}\right)}{2}$, then the function $f(x)$ is said to have property $\mathrm{P}$. Among the following functions:
$1 f(x)=\left\{\begin{array}{ll}\frac{1}{x} & (x \neq 0) \\ 0 & (x=0)\end{array} ;\right.$
$2 f(x)=x^{2}$;
$3 f(x)=\left|x^{2}-1\right|$;
$4 \quad f(x)=x^{3}$.

the function(s) that do not have property P is (are) ( ).
A: 1
B: 2
C: 3
D: 4
Answer B.
Analysis The characteristic of a function with property P is: there exists a straight line that intersects the function's graph at three points, one of which is the midpoint of the other two. From the graph, we can see that $1, 3, 4$ all have three intersection points, while 2 does not have property $\mathrm{P}$.