Which one of the following is a validity condition for theory-based tests for regression? select one: a. the variability of the points around the regression line should not differ as you slide along the x-axis. b. the scatterplot should show a positive association between the explanatory and response variables. c. the variability of the explanatory variable should be the same as the variability of the response variable. d. there should be at least 10 successes and 10 failures in the data.