Which two statements are true about unified ISSU?

Which two statements are true about unified ISSU? (Choose two.)

A.
It allows gradual implementation of new features.

B.
It reduces operating costs while delivering higher service levels.

C.
It involves network down time during software image upgrades.

D.
It requires graceful Routing Engine switchover (GRES) and nonstop active routing (NSR).

Explanation: