The correct answer is B) 5 percent methane by volume will give a reading of 100 percent L.E.L. on a combustible gas indicator. The explosive range of methane is 5% to 15% by volume in air. This means that a vapor/air mixture containing 5% methane by volume is at the lower explosive limit (L.E.L.), which would register 100% on a combustible gas indicator. Mixtures below 5% are too lean to burn, while mixtures above 15% are too rich to burn. The other answer choices are incorrect because: A) 3% methane is below the L.E.L. and would not be explosive. C) 10% methane is within the explosive range, but not at the L.E.L. D) 20% methane is above the upper explosive limit and would be too rich to burn.