27 In $\triangle A B C$, $\tan A$ is the 4th term of an arithmetic sequence with 1 as the 3rd term and 9 as the 7th term, $\tan B$ is the common ratio of a geometric sequence with 64 as the 2nd term and 1 as the 5th term, then this triangle is
A. Acute triangle
B. Right triangle
C. Obtuse triangle
D. Isosceles triangle
27 C. $\tan A=1+\frac{9-1}{7-3}-3=3, \tan B=\frac{1}{4}$, so $\tan (A+B)>0$, so
$$
C>\frac{\pi}{2} \text {. }
$$