We can ask about the algebra of the '+' operation. (a) Is it commutative, is W1 +W2

Question:

We can ask about the algebra of the '+' operation.
(a) Is it commutative, is W1 +W2 = W2 +W1?
(b) Is it associative, is (W1 +W2) +W3 = W1 + (W2 +W3)?
(c) Let W be a subspace of some vector space. Show that W +W = W.
(d) Must there be an identity element, a subspace I such that I+W = W+I = W for all subspaces W?
(e) Does left-cancellation hold: if W1 +W2 = W1 +W3 then W2 = W3? Right cancellation?