Avogadro's law states that: a. the volume of a fixed amount of gas is directly proportional to its temperature in kelvin at constant pressure. b. the total pressure of a mixture of gases is the simple sum of the partial pressure of all of the gaseous compounds. c. equal amounts of gases occupy the same volume at constant temperature and pressure. d. the volume of a fixed amount of gas is inversely proportional to its pressure at constant temperature. e. the rates of effusion of gases are inversely proportional to the square roots of their molar masses.