True or False (justify your answer by an argument or counterexample, as appropriate remember: 'X' and 'Y' cover all SL sentences, not just atomic...

16. True or False (justify your answer by an argument or counterexample, as appropriate— remember: 'X' and 'Y' cover all SL sentences, not just atomic ones)

a. If X is not a logical truth, ~X is.

b. If a set consisting of only sentence X is inconsistent, ~X must be a logical truth.

c. You can't make a valid argument invalid by adding premises.

d. If Y doesn't follow from X, then ~Y must.