Imagine a regression model where y is regressed on x1 (a dummy coded indicator representing a grouping variable with 2 levels) and x2 (a continuous numerical variable) . Which of the following is true about fitting a parallel slopes model in R (select all that apply) ?
a) The slopes are forced to be equal
b) In the model code, y is regressed on x1 + x2
c) In the model code, y is regressed on x1 * x2
d) The intercepts are forced to be equal