5. Given point $A(1,2)$, a line passing through point $(5,-2)$ intersects the parabola $y^{2}=4 x$ at two other points $B, C$, then $\triangle A B C$ is
A. Acute triangle
B. Obtuse triangle
C. Right triangle
D. The answer is uncertain
5. C Hint: Let $B\left(t^{2}, 2 t\right), C\left(s^{2}, 2 s\right), s \neq t, s \neq 1, t \neq 1, BC$'s equation is: $2 x-(s+t) y+2 s=0$, and $BC$ passes through $(5,-2)$. Thus $(s+1)(t+1)=-4, k_{A B} \cdot k_{A C}=\frac{2 t-2}{t^{2}-1} \cdot \frac{2 s-2}{s^{2}-1}=\frac{4}{(s+1)(t+1)}=-1$.