In the Cartesian plane four points P, Q, R, S have coordinates (1, 1), (4, 2), (4, 4) and (1, 4). The area of the quadrilateral PQRS is

1. (a)9
2. (b)7.5
3. (c)4.5
4. (d)Impossible to find unless the lengths of the diagonals are known