An auditor is testing accounts receivable for a client that has 1,000 customers with customer balances that range from $150 to $185,000. The auditor subdivided the receivables into three
groups: group 1 has all customers with receivable balances between $185,000 and $100,000, group 2 has all customers with receivable balances between $100,000 and $25,000, and group 3 has all customers with receivable balances less than $25,000. The auditor then randomly selects customers out of each group. This is known as:

a. Random selection.

b. Stratified sampling.

c. Haphazard selection.

d. Block selection.