Which statement about determining the empirical formula from combustion data is correct?

• A. You can always directly read oxygen content from CO2 and H2O data.
• B. Oxygen content is unnecessary for most empirical formulas.
• C. The data for carbon and hydrogen alone can completely determine the empirical formula.
• D. The data for carbon and hydrogen do not provide oxygen content directly, so oxygen is often determined by difference.

Answer: (D)