The straight lines I1, I2, I3 are parallel and lie in the same plane. A total number of m points are taken on I1, n points on I2, k points on I3. The maximum number of triangles formed with vertices at these points is
a) m + n + kC3
b) m + n + kC3 – mC3 – nC3 – kC3
c) mC3 + nC3 + kC3
d) None of these