2. (10 pts) A Regression Model includes X1, X2, ..., X20 as possible predictors to include in the final model. A bi-directional forward selection procedure based on p-values was performed where X1 was entered on the first step, X2 was entered on the second step. On the third step, X5 was entered and then on the next step X1 was removed. ~ Describe characteristics of the X's which could lead to a sequence of steps where X1 was entered first and then later removed after X5 was entered into the model.~