2.56 If the equation $m x^{2}-2(m+2) x+m+5=0$ has no real roots with respect to $x$, then the number of real roots of the equation $(m-5) x^{2}-2(m+2) x+m=0$ with respect to $x$ is
(A) 2.
(B) 1.
(C) 0.
(D) Uncertain.
(China Junior High School Mathematics League, 1989)
［Solution］Since the previous equation has no real roots, its discriminant is
$$
\Delta=4(m+2)^{2}-4 m(m+5)4 \text {. }
$$

For the subsequent equation, when $m>4$ and $m \neq 5$, its discriminant is
$$
\Delta=4(m+2)^{2}-4 m(m-5)=36 m+16>0 \text {, }
$$

indicating it has two distinct real roots.
When $m=5$, the equation becomes a linear equation, having only one real root.
Therefore, the number of real roots cannot be determined.
Hence, the correct choice is $(D)$.