If you'd like to include an extra credit question in your Activity, assign it 0 points (see How Do I Change the Point Value of a Question?) That will keep the question from being auto-scored and penalizing those who choose not to answer it.

You'll assign the extra credit points as you're reviewing student Submissions:

• Open a student Submission.
• Under "Score Type," choose "Auto + Teacher." That means that the total score for this student's work will be the calculated automatic score, plus the points you add to it.

• Enter values for the points awarded and points possible. These values are immediately saved.
• Click "Return Submission" if you'd like to return the submission to the student at this time.