Which of the following sub-sets of R3 are subspaces?
1. The set of vectors (x1, x2, x3) with x1 = x2
2. The set of vectors (x1, x2, x3) with x1 + x2 + x3 = 0
3. The set of vectors (x1, x2, x3) with x1  x2  x3
4. The set of vectors (x1, x2, x3) with x1 ⇥ x2 ⇥ x3 = 0