3. In the triangle with vertices $A(2,1), B(-1,4), C(5,3)$, the obtuse angle is
A. $\angle A$
B. $\angle B$
C. $\angle C$
D. None
3. $\mathrm{A}$ Hint: $|B C|=\sqrt{37},|A B|=\sqrt{18},|A C|=\sqrt{13}$, so $|B C|^{2}>|A B|^{2}+|A C|^{2}$.