Here's why:

* Hydrocarbons are compounds made up of only hydrogen (H) and carbon (C) atoms.

* Functionalized hydrocarbons have additional atoms or groups of atoms (functional groups) attached to the carbon chain, which give the molecule specific chemical properties.

In the case of H3C--CH3, which is also written as CH3CH3 or simply ethane, there are only carbon and hydrogen atoms. There are no functional groups present.