What should you do before you raise the domain functional level?

Your network contains an Active Directory forest. The forest contains a forest root domain named contoso.com and a child domain named eu.contoso.com. Each domain contains two domain controllers that run Windows Server 2012 R2.
The forest functional level is Windows Server 2008 R2. The domain functional level of contoso.com is Windows Server 2012 R2. The domain functional level of eu.contoso.com is Windows Server 2008 R2.
You need to raise the domain functional level of contoso.com to Windows Server 2016. The solution must minimize administrative effort.
What should you do before you raise the domain functional level?
A. Raise the forest functional level
B. Upgrade all of the domain controllers in the forest
C. Upgrade all of the domain controllers in contoso.com
D. Raise the domain functional level of eu.contoso.com