3. If $\triangle A B C$ with $\angle A B C=60^{\circ}, A C=12, B C=k$ has exactly one solution, then the range of $k$ is
A. $k=8 \sqrt{3}$
B. $0<k \leqslant 12$
C. $k \geqslant 12$
D. $0<k \leqslant 12$ or $k=8 \sqrt{3}$
3. D Hint: According to the problem, $\triangle A B C$ has two types, as shown in the figure:
Figure for Question 3
It is easy to get $k=8 \sqrt{3}$ or $0<k \leqslant 12$
This problem can also be solved using the special value method, eliminating $\mathrm{A} 、 \mathrm{~B} 、 \mathrm{C}$.