Define C(L1, L2) = {s1s2 | s1 L1, s2 L2, | s1 |=| s2 |}. That is, C(L1, L2) is a special kind of language concatenation in which only the equal-length strings in L1 and L2 are paired together. Prove that if L1 and L2 are both regular languages, then C(L1, L2) is a CFL. (Hint: Assume there exist DFAs A1 and A2 accepting L1 and L2, respectively, and use them to mathematically define a PDA that accepts C(L1, L2)).