Find 'X' in the circuit below : f1(A, B, C, D) = Σ(6, 7, 13, 14); f2(A, B, C, D) = Σ(3, 6, 7); f3(A, B, C, D) = Σ(5, 6, 7, 14, 15)

• p(0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 15)
• 0
• Σ(14)
• 1