Predict the sign of S, if possible, for each of the following reactions. If you cannot predict

Question:

Predict the sign of S, if possible, for each of the following reactions. If you cannot predict the sign for any reaction, state why.
a. C2H2(g) + 2H2(g) → C2H6(g)
b. N2(g) + O2(g) → 2NO(g)
c. 2C2H2(g) + 3O2(g) → 4CO(g) 2H2O(g)
d. 2C(s) + O2(g) → 2CO(g)