$\backslash$table$[[$Labor$,$Output$],[($Number of workers$),$Firm A$,$Firm B$,$Firm C$,$Firm D$],[1,200,200,200,200],[2,160,380,600,400],[3,120,540,1,200,600],[4,80,680,2,000,800]]$

Refer to Table $19-3.$ Which firm's production function exhibits positive but diminishing marginal product?

Firm A

Firm B

Firm C

Firm D