Which of these systems is 'underdetermined'?
x+y=1x+y=1x+y=1
x+y=1,x−y=1x+y=1, x-y=1x+y=1,x−y=1
x+y=1,x+y=2x+y=1, x+y=2x+y=1,x+y=2
x+y+z=1,x−y−z=1,y=zx+y+z=1, x-y-z=1, y=zx+y+z=1,x−y−z=1,y=z