ABCD is a quadrilateral in which $A B=A C$, $\mathrm{AD}=\mathrm{CD}=13 \mathrm{~cm}, \angle \mathrm{BAC}=20^{\circ}$ and $\angle A D C=100^{\circ}$ If $B C=12 \mathrm{~cm}$, then $A B$ is equal to
1. $20 \mathrm{~cm}$
2. $25 \mathrm{~cm}$
3. $23 \mathrm{~cm}$
4. $21 \mathrm{~cm}$