CFG is not closed under complementation , but REC is?
GATE CSE 2012 | Question: 24 (Here "Which of the following problems are decidable" is asked)
Not closed does not mean Undecidable.(https://gateoverflow.in/78060/undecidability)
C. It’s Trivial. So decidable.
D. It’s Trivial. So decidable
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