The following expression was to be realized using 2-input AND and OR gates, but by mistake all 2-input AND gates were taken as 2-input NAND gates,
$$(a.b).c + (\bar a.c).d + (\bar b.c).d + a.d$$
What is the function finally realized ?
- $1$
- $\bar a + \bar b + \bar c + \bar d$
- $\bar a + b + \bar c + \bar d$
- $\bar a + \bar b + c + \bar d$