Which of the following income items may not be excluded from gross income?

A$.$ Housing received by a medical research student at an academic health center. The monthly charge is $25\%$

of the housing's fair market value.

B$.$ Adoption expenses incurred by an employee that are reimbursed by an employer.

C$.$ Military pay received while serving in a combat zone.

D$.$ A Christmas bonus of $$100$ in cash received at the end of the year.