Two parallel metro tracks run in north-south. Metro X moves north with a speed of 60 km/h and metro Y moves south with the speed of 90 km/h. The velocity of metro Y with respect to X is
1
90 km/h along north to south.
2
120 km/h along south to north.
3
150 km/h along north to south.
4
60 km/h along south to north.