18. If the three interior angles of $\triangle A B C$ satisfy $\sin A : \sin B : \sin C = 5 : 11 : 13$ , then $\triangle A B C$\\
A. must be an acute triangle\\
B. must be a right triangle\\
C. must be an obtuse triangle\\
D. could be either an acute triangle or an obtuse triangle
\section*{III. Solution Problems (Total Score: 74 points)}