Which one of the following pair is/are correct

A. $$\frac{{\vartriangle qd}}{{\vartriangle P}} \cdot \frac{P}{{qd}}$$   - Price elasticity of demand

B. $$\frac{{\vartriangle qx}}{{\vartriangle Py}} \cdot \frac{{Py}}{{qx}}$$   - Cross elasticity of demand

C. $$\frac{{\vartriangle qs}}{{\vartriangle P}} \cdot \frac{P}{{qs}}$$   - Elasticity of supply

D. All of these

Answer: Option D