SUBSPACES (Headbang)!

Hi, I don't understand subspaces! I came across this question in homework tasks, and I'm guessing it's very important to the course?!

Which of the following subsets of R^3 are subspaces? (Justify answers)

(a) A = {(s, 0, 2s) | s Є R }

(b) B = {(x, y, z) | y ≥ z}

(c) C = {(t+1, t, 3t) | t Є R }

(d) D = {(x, y, z) | 4x + y - z = 0}

Help? Please? :-)