A PDA for L₁ pushes once per a or b and pops once per c, also checking positive a and b blocks, so L₁ is context-free. If L₂ were context-free, its inverse homomorphism under h(a)=a,h(b)=b,h(c)=cc would make {aⁿbⁿcⁿ:n≥1} context-free, a contradiction. Thus B.
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