The Market Demand Curve for Labor Is

The market demand curve for labor is

A) the horizontal summation of the firms' demand curves for labor, derived exactly the same way the product market demand curve is derived from the consumers' demand curves.
B) the vertical summation of the firms' demand curves for labor.
C) any one firm's demand curve labor multiplied horizontally by the number of firms in the labor market.
D) not the horizontal summation of the firms' demand curves for labor because higher wage rates result in a higher price for the goods produced.