Which of the following problems are decidable?
- Checking whether two regular languages are equivalent
- Checking whether two non-deterministic finite automata accept the same language
- Checking whether two context free grammars generate equivalent languages
- Checking whether a language of a context free grammar is non-empty
- $\text{I, II}$ and $\text{IV}$ only
- $\text{I, II}$ and $\text{III}$ only
- $\text{I, II, III}$ and $\text{IV}$
- $\text{I}$ and $\text{II}$ only