I already asked a similar question here but didn't get an answer that explained the reasoning. Now the question is I'm asked to find which of the circuits below preserves the logic expression, but I don't know what is meant by "preserves the logic expression". What I know is when we make the truth table just for the bottom NAND Gate for all cases (connecting the unused input to ground/+5V/E), only the case where we connect it to the ground (!(D&E&G) where G=0) we don't get 0 as an output. In the other cases we can get 1, and 0 as an output. Does this make the circuit with the ground connection, not preservative? I would appreciate it if I could get an explanation (FYI what I mean by "unused input" is the Pin 1 on the bottom NAND Gate).
Thanks in advance.