Thirteen choices enable expressing what one knows.
If nothing, a midpoint m is the best choice.
If one a, b, c answer is seen to be wrong, choose a
halfway point on an edge opposite the vertex for the answer that cannot be
possibly true, when nothing else is known about the subject.
Other answers should express belief about a topic. If not certain, but aware
of a wrong answer and belief that one of the two remaining a, b, c completions is more likely, the best thing to do is to indicate which
one by the response selection. Such choice
expresses belief that the close vertex is probably true 3/4 of the
time, the other only 1/4 likely.