18. If the three sides of a right-angled triangle are $a, b, c, \angle B=90^{\circ}$, then, the nature of the roots of the equation
$$
a\left(x^{2}-1\right)-2 c x+b\left(x^{2}+1\right)=0
$$

with respect to $x$ is ( ).
(A) It has two equal real roots
(B) It has two unequal real roots
(C) It has no real roots
(D) Cannot be determined
18. A.
$$
\begin{array}{l}
\Delta=(2 c)^{2}-4(a+b)(b-a) \\
=4\left(c^{2}+a^{2}-b^{2}\right)=0 .
\end{array}
$$