here's my question

Let V be a vector space over a field F. Suppose that Wi and W2 are two subspaces, neither of them is contained in the other. Prove or disprove the following statements: (a) W1UW2 is a subspace; (b)W1 + W2 is a subspace, where W1 + W2 is defined to be the collection of elements of the form z = x + y with x E W, and y E W2; (c) any element z in W1 + W2 can be uniquely expressed as z = x ty with x E W1 and y E W2; (d) dim(W1 + W2) = dim W1 + dim W2 if Win W2 = {0}