Which system of equations has no solutions?

1. x-3y+3z=-4
2x+3y-z=15
4x-3y-z=19

2. 3x-3y+4z=6
-3x+3y-4z=-6
6x-6y+8z=12

3. -2a-4b+6c=5
-a+3b-2c=-1
a+2b-3c=1

4. x+y=9
y+z=7
x-x=2