Wheat Farm -Diminishing Returns Set in at What Level of Input?
A)

Wheat Farm.
Below is the production function for a wheat farm, where the input is chemicals (oz) and the output is wheat (bu) .
$\begin{array}{llll}\underline{\mathrm{X}(\mathrm{oz}) } &\underline{\mathrm{Y}(\mathrm{bu}) } & \underline{\mathrm{APP}(.) } & \underline{\mathrm{MPP}(.) } \\0&0&--&--\\1 & 10 &\underline {\quad} &\underline {\quad}\\2 & 30 & \underline {\quad}&\underline {\quad} \\3 &\underline {\quad}&{13} &\underline {\quad} \\4 &{44} & \underline {\quad}& \underline {\quad} \\5 & \underline {\quad} & \underline {\quad}& 1 \\6 & 42& \underline {\quad}&\underline {\quad}\end{array}$
-Diminishing returns set in at what level of input?

A) X = 3
B) X = 4
C) X = 5
D) X = 6