Answer from cs61c-ei (Jing Chen 16669246) for Question 3
The Xs in the truth tables signal “don’t cares” meaning that the value of the input or output is not important (can be either 1 or 0). The table is incomplete. Because there are 3 inputs, there are a maximum total of 2^3 = 8 possible input combinations: 000, 001, 010, 011, 100, 101, 110, 111.