Need Explanations For True/False Questions

I know the following answers are correct, I just need to know why they are correct. If someone could give me a brief explanation as to why. It will greatly help me in understand them. TIA!!!!

1. The equation A(nxm)X = b has a unique solution for all b contained in R^n if and only if the rank of A is n. (A is an n x m matrix.) False. But why?

2. Ax = 0 has more than one solution if and only if the columns fo A form a linearly depenedent set. True, but why?

3. If Ax = b has a general solution u + x2v + x4w then AB = 0 where B is the matrix with columns v and w. (x2 and x4 are x sub 2 and x sub 4) True, but why??

4. If A and B are invertible n x n matrices, then A + B is invertible. False, but why?????