Question

Which of the following problems are decidable? 

1) Does a given program ever produce an output? 

2) If L is context-free language1 then1 is L also context-free? 

3) If L is regular language/ then1 is L also regular? 

4) If Lis recursive language/ then1 is L also recursive?

(A)1,2,3,4

(B) 1, 2 

(C) 2,3,4

(D) 3,4

Answer 1

(D) 3, 4

CFL's are not closed under complementation. Regular and recursive languages are closed under complementation.