What was a key finding of the study by Dixon et al. (2018) regarding the relationship between relational responding and IQ scores?
a. Relational responding had no impact on IQ scores in individuals with disabilities
b. Relational training did not lead to any improvements in IQ scores
c. Multiple exemplar-based relational frame training increased IQ for some participants by at least one standard deviation
d. Verbal behavior assessments were more strongly correlated with IQ scores than relational responding