According to the credit utlization ratio, using less than$\_\_\_\_\_\_\_\%$ of one's available credit will $\_\_\_\_\_\_\_\_\_\_\_\_\_$ one's credit score over time, even if the credit card balance is paid in full each month to avoid interest charges.

a$.$

$50\%$; raise

b$.$

$10\%$; lower

c$.$

$75\%$; have no impact on

d$.$

$30\%$; raise