To know whether the rich are getting richer and the poor getting poorer, it is necessary to compare

(a)

the wholesale price index over different periods of time for different regions

(b)

the distribution of income of an identical set of income recipients in different periods of time

(c)

the distribution of income of different sets of income recipients at a point of time

(d)

the availability of foodgrains among two sets of people, one rich and the other poor, over different periods of time