linear algebra urgent help please

true or false and why?

1) every square matrix can be written as the product of elementary matrices.

2) The inverse of an elementary matrix is an elementary matrix.

3) Every system with 3 equations in 4 unknowns has at least one solution.

4) A square matrix is defined to be stochastic if all its elements are nonnegative and the sum of the elements in any given column is 1.

Prove that the product of any two stochastic matrices is stochastic.

please help me urgently. Thank you very much.