Wheat Farm -What Is the TPP of Using the 3rd Unit of Farm

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

A) 9
B) 20
C) 13
D) 39