Small increases in customer retention can result in large increases in profit because $$

A$.$ returning customers always buy more units relative to new customers

B$.$ It costs four to five times more to get a new customer than it does $10$ retain one

C$.$ existing customers can be serviced with less expensive staff members

D$.$ new customers are fickle and frequently swith vendors

E$.$ retention is very easy given most customers are reluctant to change suppliers