subspace

Determine which of the following are subspaces of R^3; for those which are subspaces, find a basis and write down the dimension of the subspace:

i) {(x, y, z) E R^3 : x = y = 0}
ii) {(x, y, z) E R^3 : y = 2z = 4}
iii) {(x, y, z) E R^3 : y = 3x + 4z}
iv) {(x, y, z) E R^3 : [5^(1/2)]y + [7^(1/2)]z = 0}

i.e. "E" denotes "in the interval of"