77. On a hydrocarbon flammability chart the line which extends from 0% to 21.8% oxygen, lying tangent to the flammability range, is called the __________.

The line on a hydrocarbon flammability chart extending from 0% to 21.8% oxygen, tangent to the flammability range, is the critical dilution line. This line represents the limiting path for dilution that avoids entering the flammable zone and is a standard term in hydrocarbon flammability diagrams.