An ideal gas equation can be written as $P={\rho RT\over M_0}$ where $\rho$ and $M_0$ are respectively,

(1) Number density, mass of the gas

(2) Mass density, mass of the gas

(3) Number density, molar mass

(4) Mass density, molar mass

(1) Number density, mass of the gas

(2) Mass density, mass of the gas

(3) Number density, molar mass

(4) Mass density, molar mass