Why is the Marginal cost curve downward sloping?

Marginal cost curve is u-shaped curve, this is due to law of variable proportion(return to factors), firstly, there is an increasing return (i.e, decreasing cost) then there is a stage of constant returns (i.e, constant cost) then lastly comes the stage of decreasing returns (i.e increasing cost), thats why marginal cost curve first slopes downward and then slope upward and become u-shaped.