The explosive range of a mixture of flammable vapors and air lies between the lower and upper explosive limits. These limits are specified as a percentage of __________.

Correct Answer: Option D — flammable vapor by volume in air

Explosive limits define the concentration range of flammable vapors in air that will ignite if an ignition source is present. These limits are expressed as a percentage of the total volume of flammable vapor mixed with air. Understanding these percentages is vital for gas-freeing and hot work safety.