Cesium chloride density gradient centrifugation is commonly used for the separation of DNA molecules. The buoyant density, $\rho$ of a double stranded $Cs^+DNA$ is given by the equation $\rho = 1.66 + 0.098X_{G+c}$ where $X_{G+C}$ denotes

1. total number of $G$ and $C$
2. mole fraction of $G+C$
3. number of $GC$ repeats
4. ratio of $G+C$ to $A+T$ content