convex set and extreme point

An nxn matrix is called a permutation matrix if there is exactly one 1 in each row and column, and all other entries are 0.

An nxn matrix A=(aij) is called doubly stochastic if aij>=0 for all 1<=i,j<=n, and the sum of the entries in any row or column is 1.

Let C denote the set of all doubly stochastic nxn matrices.

(a) SHow that C is convex.

(b) Show that if P is an nxn permutation matrix, then P is an extreme point of the convex set C(the converse is also true).

Can anyone help? I have spent like 4 hours on this question and am going crazy. Thanks very much guys. :)