An account balance is $300,000 and there are 25 items in the account, six of which have balances that equal or exceed $15,000. The auditor plans to use a monetary-unit sampling plan with systematic sample selection. To ensure that all accounts with balances of at least $15,000 are selected, the sampling interval should be .

Please provide explanation

a.6

b. 20

c. 12,000

d. 15,000