PLS HELP! What are examples of a chemical reaction?

Select all that apply.

a. O2(g) + O(g) → O3(g)
b. 2H2O(l) → 2H2(g) + O2(g)
c. CH4(g) + 2O2(g) → CO2(g) + 2H2O(g)
d. CO2(s) → CO2(g)