21) Consider the unstable process transfer function Gp=s2K and the characteristic equation of a simple control system: 1+GcGp=0. (a) What is the stability criterion if we use a PD controller? How is it different from a proportional controller? (b) What is the stability criterion if we use a PI controller? (c) Sketch the root-locus plots for parts (a) and (b). (d) Derive the ultimate gain and the ultimate frequency in parts (a) and (b)