9. $\alpha, \beta$ one is an acute angle, one is an obtuse angle, four students, A, B, C, D, when calculating $\frac{1}{5}(\alpha+\beta)$, obtained the results $17^{\circ}, 42^{\circ}, 56^{\circ}, 73^{\circ}$ respectively, among which there is indeed a correct result, then the student who calculated correctly is ( ).
(A) A
(B) B
(C) C
(D) D
Answer: B.
Solution: Let $\alpha$ be an acute angle, and $\beta$ be an obtuse angle, then $0^{\circ}<\alpha<90^{\circ}$, $90^{\circ}<\beta<180^{\circ}$, 
$$
54^{\circ}, 73^{\circ}>54^{\circ} \text {, }
$$

Therefore, $\frac{1}{5}(\alpha+\beta)$ should result in $42^{\circ}$,

That is,
Yi's calculation is correct.

Hence, the answer is B.