A first order gaseous phase reaction is catalysed by a non-porous solid. The kinetic rate constant and the external mass transfer co-efficients are k and kg respectively. The effective rate constant (keff) is given by________________?
A. keff = k + kg
B. keff = (k+ kg)/2
C. keff = (kkg)1/2
D. 1/keff = 1/k + 1/kg