Part of a Research Study Entails That Research Subjects Complete

Part of a research study entails that research subjects complete a printed form with checkboxes. The researcher decides ahead of time that any item that has more than one response checked will be treated as unanswered. What are logical ways the researcher can maximize properly answered items, without skewing the data?

A) Make a rule that if there are two checked replies, only the first one will be counted.
B) When the researcher hands the form to the subjects, the researcher should specify "One answer and one answer only for each of these items, please, or I won't be able to include the double-answered ones in my report."
C) Glance at the forms as they are about to be handed in, and if two boxes for an item are checked, ask the subject to address this.
D) In the printed instructions at the top of the form, be sure to state that if a question has more than one response checked, that item will be treated as unanswered.
E) Eliminate from the entire study any subject who double-selects an item.