The following expression was to be realized using $2$-input AND and OR gates. However, during the fabrication all $2$-input AND gates were mistakenly substituted by $2$-input NAND gates. $(a.b).c + (a'.c).d + (b.c).d + a. d$
What is the function finally realized ?
- $1$
- $a' + b' + c' + d'$
- $a' + b + c' + d'$
- $a' + b' + c + d'$