Oxygen gas, O₂, is very reactive while argon gas is inert. But why do both oxygen gas and argon gas have zero standard enthalpies of formation?

• A. Because oxygen gas and argon gas are both gases of pure elements, their standard enthalpy of formation is zero.
• B. Because oxygen gas and argon gas are both reactive substances of pure elements, their standard enthalpy of formation is zero.
• C. Because oxygen gas and argon gas are both standard states of pure elements, their standard enthalpy of formation is zero.
• D. Because oxygen gas and argon gas are both non-reactive substances of pure elements, their standard enthalpy of formation is zero.

Answer: (C)