4. Which of the following set is not a subspace of R³? a) {(x, y, z) € R³ | 3x+y+2= = 0} b) {(x, y, z) € R³ | y=x+=} c) {(x, y, z) € R³ | 4x = 3y = 2= } d) {(x, y, z) € R³ | x+y+z=1} L L