Which one of the following statements is correct?
a. The correlation is always the same as the slope of the regression line.
b. The mean of the residuals from the least-squares regression line is 0 only when r = 0.
c. The correlation is the percentage of points that lie in the quadrants where x and y are both above the mean or both below the mean.
d. The correlation is inappropriate if a U-shaped relationship exists between x and y.