Within a salesperson's prospect pool, which group is the most important prospect for future sales?

A) Leads
B) Referrals
C) Centers of influence
D) Customers
E) Orphans

Customers are the most important prospects for future sales because they have already made a purchase and are more likely to buy again if they had a positive experience. Repeat business and upselling to existing customers are often more cost-effective than acquiring new ones.