Suppose that L1 is a regular language and L2 is a context-free language. Which one of the following languages is NOT necessarily context-free?
(A) L1∩L2
(B) L1⋅L2
(C) L1−L2
(D) L1∪L2
Answer: (C)
Explanation: CFLs (context-free languages) are not closed under complementation property, so complement of a CFL need not be CFL.
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