3. Let real numbers $a, b, c$ satisfy
$$
\left\{\begin{array}{l}
a^{2}-b c-8 a+7=0 \\
b^{2}+c^{2}+b c-6 a+6=0
\end{array}\right.
$$

Then the range of values for $a$ is
(A) $(-\infty,+\infty)$;
(B) $(-\infty, 1] \cup[9,+\infty)$;
(C) $(0,7)$;
(D) $[1,9]$.
3. (D)
$$
\begin{array}{l}
\left\{\begin{array}{l}
a^{2}-b c-8 a+7=0 \\
b^{2}+c^{2}+b c-6 a+6=0,
\end{array}\right. \\
\text { we get } \quad\left\{\begin{array}{l}
b c=a^{2}-8 a+7 \\
b^{2}+c^{2}+b c=6 a-6 .
\end{array}\right. \\
\therefore \quad b^{2}+c^{2}-2 b c=-3\left(a^{2}-10 a+9\right) \text {, } \\
(b+c)^{2}=-3(a-1)(a-9) \geqslant 0 \text {, } \\
\therefore \quad 1 \leqslant a \leqslant 9 . \\
\end{array}
$$