9. In $\triangle A B C$, the lengths of the three sides are $a, b, c$. If $\frac{1}{a}, \frac{1}{b}, \frac{1}{c}$ form an arithmetic sequence, then the angle opposite to $b$ is ( ).
A. Acute angle
B. Right angle
C. Obtuse angle
D. Cannot be determined
9. A ;

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