Let the marginal product of labor and capital be denoted by MPL and MPK respectively. The price of labor and capital are given by PL and PK respectively. Which of the following equations gives the condition for producing a given level of output at the least cost combination of inputs?

a. PL × PK = MPL × MPK
b. PL × MPL = PK × MPK
c. MPL ÷ PK = MPK ÷ PL
d. PL ÷ PK = MPL ÷ MPK

D