There are $M$ points on one straight line $A B$ and $n$ points on another straight line $A C$ none of them being $A$. How many triangles can be formed with these points as vertices?
- $m n(m+n-2)$
- $\frac{1}{2} m n(m+n-2)$
- $\frac{1}{2} m n(m+n-1)$
- $m n(m+n-1)$
