Question 4 In $\triangle A B C$, the sides opposite to angles $A, B, C$ are $a, b, c$ respectively, and $a^{n}+b^{n}=c^{n}(n \geqslant 3)$. Then the shape of $\triangle A B C$ is $(\quad)$.
A. Right triangle
B. Acute triangle
C. Obtuse triangle
D. Right or obtuse triangle
Solution: Clearly, $c$ is the largest side. Since $\left(\frac{a}{c}\right)^{n}+\left(\frac{b}{c}\right)^{n}=1$ ($n \geqslant 3$), we have
$$\left(\frac{a}{c}\right)^{2}+\left(\frac{b}{c}\right)^{2}>\left(\frac{a}{c}\right)^{n}+\left(\frac{b}{c}\right)^{n}=1$$

Thus, $a^{2}+b^{2}>c^{2}$. By the cosine rule, the largest angle $C$ of the triangle is acute, hence $\triangle A B C$ is an acute triangle. The correct choice is B.