The Supply Curve of Labor to a Competitive Firm Is

The supply curve of labor to a competitive firm is:

A) upward-sloping because of the law of diminishing marginal returns.
B) downward-sloping because the supply of labor increases as wages increase.
C) perfectly inelastic because the stock of labor is fixed in the long run.
D) horizontal because the firm can hire as many workers as it wants at the market wage.