Paper chromatography can be used to separate a mixture of soluble substances. The $R_f$ value can then be used to identify each substance. Which formula is used to calculate the $R_f$ value?
- A$R_f = \frac{\text{distance travelled by solvent}}{\text{distance travelled by substance}}$
- B$R_f = \frac{\text{distance travelled by substance}}{\text{distance travelled by solvent}}$
- C$R_f = \text{distance travelled by solvent} - \text{distance travelled by substance}$
- D$R_f = \text{distance travelled by solvent} \times \text{distance travelled by substance}$