Which of the following statements are true ?

P : Principal of optimality holds on Spanning tree problem.

Q : Root of s depth first search tree for a graph G is an articulation point if it has more than one child.

R : Greedy method gives optimal results if selection procedure of input is optimal.

S : n files with sizes k1, k2,k3,. … , kn can be merged into a single file in \(\mathop \sum \limits_{i = 1}^n {k_i}\) times.