Which statement correctly describes unified in-service software upgrades (ISSU)?

Which statement correctly describes unified in-service software upgrades (ISSU)?
A . Unified ISSU is built upon graceful-restart extensions to allow upgrades with no impact to the data plane.
B . Unified ISSU is supported on systems with single or dual Routing Engines.
C . Unified ISSU permits both upgrades and downgrades with no control plane disruption and is based on the graceful Routing Engine switchover (GRES) feature.
D . Unified ISSU is based on nonstop-routing (NSR) and graceful Routing Engine switchover (GRES), and permits software upgrades only.

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Answer: D