. 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 mm and 5 mm respectively. The numbers of teeth in P, Q and R are 20, 40 and 25 respectively. The number of teeth in S is
(a) 23 (b) 3 (c) 50 (d) 53