If in a single-subject study, what does it mean if the percentage of nonoverlapping data (PND) is 50%?

a) There is a 50% chance that the participant will exhibit the target behavior on any given observation.
b) The result is statistically significant.
c) Half the responses in the treatment condition are more extreme than the most extreme response in the baseline condition.
d) Half the responses made by the participant are "good" responses and the other half are "bad" responses.