Niles and Marsha adopted an infant boy $($a U$.$S$.$ citizen$).$ They paid $$15,500$ in $2020$ for adoption$-$related expenses. The adoption was finalized in early $2021.$ Marsha received $$4,000$ of employer$-$provided adoption benefits. For part $($a$),$ assume that any adoption credit is not limited by modified AGI or by the amount of tax liability.

A$.$What amount of adoption credit, if any, can Niles and Marsha take in $2021?$

B$.$Using the information in part $($a$),$ assume that their modified AGI was $$223,000$ in $2021.$ What amount of adoption credit is allowed in $2021?$