CO₂ Acts as an Acid in the Reaction CaO(s)

Multiple Choice

CO₂ acts as an acid in the reaction CaO(s) + CO₂ ⇌ CaCO₃(s) because it ________.

A) turns blue litmus red
B) reacts with a metal
C) is a proton donor
D) is an electron-pair acceptor
E) is a gas containing a non-metal