10. Let $f(x)=x^{2}+b x+c$. If the equation $f(x)=x$ has no real roots, then the equation $f(f(x))=x(\quad)$.
(A) has four distinct real roots
(B) has two distinct real roots
(C) has one real root
(D) has no real roots
10. D.

From $f(x)=x$ having no real roots, we know that the graph of the quadratic function $f(x)=x^{2}+b x+c$ is above the line $y=x$, i.e., $f(x)>x$.
Therefore, $f(f(x))>f(x)>x$.