Marginal rate of factor substitution $$\left( {\frac{{\vartriangle {{\text{X}}_1}}}{{\vartriangle {{\text{X}}_2}}}} \right)$$  is equivalent to

A. $$\frac{{{\text{P}}{{\text{X}}_1}}}{{{\text{P}}{{\text{Y}}_1}}}$$

B. $$\frac{{{\text{P}}{{\text{X}}_2}}}{{{\text{P}}{{\text{Y}}_2}}}$$

C. $$\frac{{{\text{P}}{{\text{X}}_2}}}{{{\text{P}}{{\text{Y}}_1}}}$$

D. $$\frac{{{\text{P}}{{\text{X}}_1}}}{{{\text{P}}{{\text{Y}}_2}}}$$

Answer: Option C