Using only NAND gates, which sequence correctly realizes f = (A + BC)′?

Using only NAND gates, which sequence correctly realizes f = (A + BC)′?

• A. N1 = A NAND A = A', N2 = B NAND C = (BC)', N3 = N1 NAND N2 = (A' (BC)')', N4 = N3 NAND N3 = (A + BC)' = f
• B. N1 = A NAND B = (AB)'; N2 = C NAND C = C', N3 = N1 NAND N2 = (AB)' (C')', N4 = N3 NAND N3 = f
• C. N3 = N1 NAND N2 = (A' (BC)')' = A' + BC
• D. N4 = N3 NAND N3 = (A + BC)'

Answer: (D)