Rewrite the following sets using set-builder notation:

(a) {1,3,5,7,9,11,...}

(b) {(1,4),(2,8),(3,12),(4,16),(5,20),...}

(c) {(1,1),(2,1),(2,2),(3,1),(3,2),(3,3),(4,1),(4,2),(4,3),(4,4),...}