93 If there exists a real number $a$ such that the equation $x \sqrt{a(x-a)}+y \sqrt{a(y-a)}=\sqrt{|\lg (x-a)-\lg (a-g)|}$ holds in the real number range, then the value of $\frac{3 x^{2}+x y-y^{2}}{x^{2}-x y+y^{2}}$ is
A. 3
B. $\frac{1}{3}$
C. 2
D. $\frac{7}{1993}$
93 B. According to the problem, we have
$$
\left\{\begin{array}{l}
a(x-a) \geqslant 0, \\
x-a>0, \\
a(y-a) \geqslant 0, \\
a-y>0 .
\end{array}\right.
$$

Thus, we know $a=0$. Substituting $a=0$ into the original equation, we get $x=-y$, hence
$$
\frac{3 x^{2}+x y-y^{2}}{x^{2}-x y+y^{2}}=\frac{3 \cdot(-y)^{2}+(-y) \cdot y-y^{2}}{(-y)^{2}-(-y) \cdot y+y^{2}}=\frac{1}{3} .
$$