Mike and Sandy are two woodworkers who both make tables and chairs. In one month, Mike can make $4$ tables or $20$ chairs, where Sandy can make $6$ tables or $18$ chairs. What is the opportunity cost of $1$ chair?Select one:a$\mathrm{.}1/3$ table for Mike and $1/5$ table for Sandyb$\mathrm{.}3$ tables for Mike and $5$ tables for Sandyc$\mathrm{.}1/5$ table for Mike and $1/3$ table for Sandyd$\mathrm{.}5$ tables for Mike and $3$ tables for Sandy