Which of the following is the correct ( unbalanced) equation for the reaction that takes place when solid phosphorus combines with oxygen gas to form diphosphorus pentoxide?
A. P(s) + O2(g) --> P2O5(s)

B. P2O5(s) --> P2(s) + O2(g)

C. P(s) + O(g) --> P5O2(g)

D. P(s) + O2(g) --> PO2(g)