A spherical wave travels such that a crest occurs at radius \(r_{1}\) at instant \(t_{1}\). The same crest has a radius \(r_{2}=2 r_{1}\) at \(t_{2}=2 t_{1}\).

(a) What is the wave speed? 

(b) Where is the crest at \(t_{3}=3 t_{1}\) ?

(c) What is the ratio (energy per unit area at \(\left.t_{3}\right) /\left(\right.\) energy per unit area at \(t_{1}\) )?