Which of the following accurately describes the certificate of deposit investment strategy known as laddering?

a$)$ Redeeming a certificate of deposit and reinvesting in a new certificate when interest rates increase

b$)$ Purchasing multiple certificates of deposit $($CDs$),$ rather than just one, with equally spaced terms of maturity

c$)$ Purchasing certificates in progressively increasing deposit amounts

d$)$ Immediately purchasing another certificate as one certificate matures