Which of the following set of measures are appropriate in monitoring 'customer retention'?

Select one:

a.Proportion of sales from existing customers, number of repeat purchases, sales growth from customers using loyalty cards

b.Customer satisfaction rating, number of repeat purchases, customer acquisition costs

c.Customer satisfaction rating, sales growth from customers using loyalty cards, customer profitability per quarter

d.Proportion of sales from existing customers, market share, profit attributable to customers in the 21-30 age group