$9 \cdot 37$ There exists an integer $x$, such that a triangle with side lengths $10, 24, x$ has all acute angles. How many such $x$ are there?
(A) 4.
(B) 5.
(C) 6.
(D) 7.
(E) More than 7.
(39th American High School Mathematics Examination, 1988)
[Solution] If the triangle is an acute triangle, by the cosine theorem we know
$$
\left\{\begin{array}{l}
24^{2}<10^{2}+x^{2} \\
x^{2}<10^{2}+24^{2} .
\end{array}\right.
$$

That is, $476<x^{2}<676$, which gives $x=22,23,24,25$. Therefore, the answer is $(A)$.