$17 \cdot 176$ The ratio of the three sides of a certain triangle is $6: 8: 9$, then
(A) This triangle is an obtuse triangle.
(B) The three angles are in the ratio $6: 8: 9$.
(C) This triangle is an acute triangle.
(D) The angle opposite the largest side is twice the angle opposite the largest side.
(E) None of the above.
[Solution] Note that $(6 t)^{2}+(8 t)^{2}=(10 t)^{2}>(9 t)^{2}$, which means the angle opposite the longest side of the triangle is acute.
Therefore, the answer is $(C)$.