The term flammable liquid means any liquid which gives off flammable vapors (as determined by flashpoint from an open-cup tester, as used for test of burning oils) at or below a temperature of 80 °F. Flammable liquids are referred to by grades as follows:
(a) Grade A. Any flammable liquid having a Reid vapor pressure of 14 pounds or more.
(b) Grade B. Any flammable liquid having a Reid1 vapor pressure under 14 pounds and over 81/2 pounds.
(c) Grade C. Any flammable liquid having a Reid1 vapor pressure of 81/2 pounds or less and a flashpoint of 80 °F. or below.