How often is a release upgrade required for RISE with SAP S/4HANA Cloud, private edition?

How often is a release upgrade required for RISE with SAP S/4HANA Cloud, private edition?
A . When the base release falls out of mainstream maintenance
B . When the latest base release is launched
C . When the current base release has only 12 months of mainstream maintenance remaining
D . There is no time requirement for upgrades

Answer: B