Answer from cs61c-bg (Spencer Ahrens 16187781) for Question 3
in general, X's mean that a value is irrelevent if it is an input value, or that it's result is meaningless if it is an output.

o = ~a + ~c

The combinations are all satisfied; o does not depend on b, it is always 1 if a is 0, and if a is one, then it is important to know what the value of c is.