A Reason Why a Firm's Demand for Labour Curve Slopes

A reason why a firm's demand for labour curve slopes downward is that,holding everything else constant,

A) the extra cost of hiring additional units of labour increases as a firm hires more units of labour.
B) as more labour is hired, labour's marginal product falls due to diminishing marginal returns.
C) the firm's demand curve for the product produced by the labour is downward sloping.
D) each additional unit of labour hired is less efficient than previously hired units.