Combinatorial proofs

Prove the following using combinational proofs (i.e not using the formulas)

C(n, k) = C(n - 1, k) + C(n - 1, k - 1) C(n, k) = C(n - 1, k) + C(n - 1, k - 1)

C(n, k) = C(n, n - k).

C(n, r)C(r, k) = C(n, k)C(n-k, r-k)

 k·C(n, k) = n·C(n - 1, k - 1)