Which of the following problems are decidable?
1) Does a given program ever produce an output?
2) If L is a context-free language, then, is ~L also context-free?
3) If L is a regular language, then, is ~L also regular?
4) If L is a recursive language, then, is ~L also recursive?
(A) 1, 2, 3, 4
(B) 1, 2
(C) 2, 3, 4
(D) 3, 4