3. Let Tn be the set of all the rooted trees on the vertex set [n] with the root 1. A leaf of a tree T is a non-root vertex that has degree 1. (a) Can a rooted tree in Tn have no leaf? Justify your answer. (b) How many rooted trees in Tn have exactly one leaf? Describe those trees. (Note that the root vertex 1, by definition, is not a leaf. In the question we assume n 2. )