5. Given $a \neq \pm 1$, the triangle formed by the three intersection points of the hyperbola $x y=a$ and the parabola $y=x^{2}+a x-1$ is ( ).
A. Right triangle
B. Equilateral triangle
C. Obtuse triangle
D. Scalene acute triangle
5. A. $\left\{\begin{array}{l}x y=a \\ y=x^{2}+a x-1\end{array}\right.$ gets three intersection points $A(1, a), B(-1,-a), C(-a,-1)$. Since $k_{A C} \cdot k_{\mathrm{BC}}=-1$, hence $A C \perp$ $B C$, so the answer is A.