Can I complete only one portion of the ballot?

Yes. This is called an under-voted ballot and will automatically be processed by the tabulator. In a race with only one selection to be made (i.e., Mayor or Ward Councillor), the tabulator will not count a vote for any candidate in an under-voted race; it will only report that an under-vote has occurred for that specific office.  In a race with more than one selection to be made (e.g., School Board Trustee), the tabulator will only count votes for the selections made.