Prove the existence of regular $C$ so that: $A \prec C \prec B $
Hint: Note that $B \setminus A$ is regular by closure properties of $\mathsf{REG}$. 


   Since $B \setminus A$ is also infinite, you can find an infinite language $D \in \mathsf{REG}$ so that $D \prec B \setminus A$.