Which of the following are true assumptions underlying linear regression? (1) for each value of x, there is a group of y values that is normally distributed. (2) the means of these normal distributions of y values all lie on the regression line. (3) the standard deviations of these normal distributions are equal?
a)only (1) and (2)
b)only (1) and (3)
c)only (2) and (3)
d)all of them