A gaseous sample of pure methane, CH4, is found to occupy a volume of 20.0 L at 25℃ and 1 atm pressure.  What volume of O2, at the same temperature and pressure, should be just sufficient to permit complete combustion of the methane?
    10.0 L
    20.0 L
    30.0 L
    40.0 L
    None of the above is correct.