Answer from cs61c-bx (Tuan Ha 16989532) for Question 3
X = don't care, it means 0 or 1 or anything else.
o = !a + a.!c
There are four cases for the 0XX, 2 cases for 1X0, and 2 cases for the 1X1. So, there are a total of 8 different cases. Since there are 3 inputs, the number of possible input combinations is 8. Therefore, the table is complete.