A proof of Rado's Theorem via principal extension

We give a new proof of Rado's Theorem that, given a partition (X1,…,Xn) of the ground set of a matroid M, there is an independent set of M containing exactly one element from each of the sets (X1,…,Xn) if and only if for each subset J of {1,…,n} we have r(∪j∈JXj)≥|J|.