Why Does the Marginal Revenue Product (MRP) Curve Slope Downward

Why does the marginal revenue product (MRP) curve slope downward for a perfectly competitive firm?

A) Because MRP = MR x MPP.After some point, as more of a factor is employed, the lower its MPP is; thus MRP declines.
B) Because MRP = MFC x MPP.After some point, as more of a factor is employed, the lower its MFC is; thus MRP declines.
C) Because MRP = MR x MPP.After some point, MR declines for a product price taker; thus, MRP declines.
D) Because MRP = MFC x MR.After some point, MFC and MR decline; thus, MRP declines.