What is the ratio of specific speeds (Ns)A : (Ns)B?

Question

Two centrifugal pumps ‘A’ and ‘B’ operate at their maximum efficiencies at 1000 rpm and 500 rpm respectively. Against the same delivery head, pump ‘A’ discharge 1 m3/s and pump B discharge 4 m³/s respectively. What is the ratio of specific speeds (N_s)A : (N_s)B?

a. 1 : 2                         b. 1 : 1

c. 1 : 4                         d. 4 : 1