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702.21a Cumulative upkeep is a triggered ability that imposes an increasing cost on a permanent. "Cumulative upkeep [cost]" means "At the beginning of your upkeep, if this permanent is on the battlefield, put an age counter on this permanent. Then you may pay [cost] for each age counter on it. If you don't, sacrifice it." If [cost] has choices associated with it, each choice is made separately for each age counter, then either the entire set of costs is paid, or none of them is paid. Partial payments aren't allowed.
Example: A creature has "Cumulative upkeep {W} or {U}" and two age counters on it. When its ability next triggers and resolves, the creature's controller puts an age counter on it and then may pay {W}{W}{W}, {W}{W}{U}, {W}{U}{U}, or {U}{U}{U} to keep the creature on the battlefield.