17-185 A triangle with side lengths $a, b, c$ satisfies:
$$
\frac{1}{a}-\frac{1}{b}+\frac{1}{c}=\frac{1}{a+b-c},
$$

then this triangle is
(A) an equilateral triangle.
(B) an isosceles triangle.
(C) a scalene triangle.
(D) a right triangle.
(1st "Five Sheep Cup" Junior High School Mathematics Competition, 1989)