Keith was divorced from Barbara in $2018.$ Barbara received custody of their two children. Keith was ordered to pay $$1,000$ per month to Barbara until the youngest child reaches age $18.$ At that time, the payments are to decrease to $$400$ per month. What portion, if any, is deductible by Keith as qualifying alimony?

A$)$

$$500$

B$)$

$$1,000$

C$)$

$$400$

D$)$

$$0$