Which of the given subsets of R3 are subspaces?

Question

(a) The set of all vectors of the form
[a;b;0]

b)The set of all vectors of the form

[a;b;c]
where a > 0.

c) The set of all vectors of the form
[a;c;c]

d) The set of all vectors of the form
[a;b;c]
where
2a − b + c = 5.