I have gotten two messages demonstrating a state of confusion about 
the fundamental difference between circuits in figures 1a and 1b.  
The question expressed concern that they had no clue about what I am 
getting at with this problem.  They also said the hint is not clear.

So, two things I want to say:

Thing 1: yes, there is something VERY VERY major fundamentally 
different about these two circuits.  And, I would like you to be 
able to see it for yourself.

Thing 2: Perhaps I need to expand the hint a little.  The hint says:

   What happens when the voltage at input A goes from 0 to 1 in both circuits?

When we say "what happens?" we usually mean (not always, but a good 
first place to look) "what happens at the output?," in this case D.

Suppose A=0, B=1, C=0.  What is D in the two circuits?  If A goes 
from 0 to 1, what happens to the output in each of the two circuits?  
Why?  Does that help you see the "fundamental difference" between 
the circuits in 1a and 1b.

Yale Patt