$3\mathrm{.}7($Based on Boykin $[20])$ A chemical production company annually produces $500$ million pounds of the chemical maleic anhydride using four different reactors. Each reactor can be run on only one of the four settings. The following table gives the annual cost $($thousands of dollars$)$ and production $($in millions of pounds$)$ for each reactor and each setting.

A reactor can only be run on one setting for the entire year. How can they meet its annual demand at a minimum cost?