A car X starts from a town A and goes towards point C. At the same time, car Y starts from a town B(lying between A and C) and goes towards town C. X catches up with Y at a town M between B and C. A car Z starts from town A when the cars X and Y meet. The cars X and Z meet at a point N where CN=AB. The cars Y and Z meet at a point O where MO= 20 miles. Then the length of AB given AC = 140 miles and BM = 20 miles is

1) 20 miles
2) 30 miles
3) 60 miles
4) 40 miles