In a reverted gear train, two gears \(P\) and \(Q\) are meshing, \(Q-R\) is a compound gear, and \(R\) and \(S\) are meshing. The modules of \(P\) and \(R\) are \(4 \mathrm{~mm}\) and \(5 \mathrm{~mm}\) respectively. The number of teeth in \(P, Q\) and \(R\) are 20, 40 and 25 respectively. The number of teeth in \(S\) is

(a) 23

(b) 35

(c) 50

(d) 53 .