See the map below showing the position of two factories and a village. People in the village suspect that one or both of the factories are polluting the river from which they use the water for their daily needs. However the factories claim that the river is already polluted before it reaches them and they are not responsible for the pollution. (Note the direction of the river's flow.)





Which is the minimum number of points at which samples of water should be taken to identify whether and which factory is polluting?