There are M points on one straight line AB and n points on another straight line AC none of them being A. How many triangles can be formed with these points as vertices?
1
mn(m + n - 2)
2
\(\frac{1}{2}\)mn(m + n - 2)
3
\(\frac{1}{2}\)mn(m + n - 1)
4
mn(m + n - 1)