$23 \cdot 27$ A rectangular box has a total edge length of 140, and the length of its diagonal is 21. Then the surface area of the box is 
(A)776.
(B) 784 .
(C) 798 .
(D) 800 .
(E) 812 .
(47th American High School Mathematics Examination, 1996)
[Solution] Let the length, width, and height of the rectangular prism be $a, b, c$. According to the problem,
$$
\left\{\begin{array}{l}
4(a+b+c)=140, \\
a^{2}+b^{2}+c^{2}=21^{2} .
\end{array}\right.
$$

Thus, $a+b+c=35$.
$$
\begin{aligned}
\therefore \quad S_{\text {surface }} & =2(a b+b c+c a) \\
& =(a+b+c)^{2}-\left(a^{2}+b^{2}+c^{2}\right) \\
& =35^{2}-21^{2}=784 .
\end{aligned}
$$

Therefore, the answer is $(B)$.