Answer from cs61c-al (Phillip Chin 16156822) for Question 3
Xs are don't cares, which mean they're values don't matter. 

o = !a + a.!c
It would appear that since they don't matter, they would not have to be added to the table.

This table is not complete. For every defined value, there could be the opposite, so no every possibility isn't there, but a lot are.