digital circuit ....right now

QB) Can you answer following questions after analyzing the digital circuit and the given truth table for the MSI chip 74x138 in the Figure 2:

1. a)  What is function of this circuit?

2. b)  Please draw the circuit implement the function N3N2N1N0’ + N3’N2N1’N0’ using Figure 2.

3. c)  Suppose following changes are made in Figure 2: G2B of U2 is connected to N3 (instead of

   logic ‘0’); G1 of U2 is connected to logic ‘1’ (instead of N3); N3 is also connected to G1 of U1(G1 of U1 is no longer connected to 5V through R and G2A of U1 is connected to logic ‘0’ instead of N3) . What will be the impact of these changes? How will you implement the above function now N3N2N1N0’ + N3’N2N1’N0’?