in an experiment, a researcher compares three different drugs (say, A, B, and C) used to reduce high blood pressure. for each drug, 15 patients received the drug. The response variable is systolic blood pressure (Y). in addition to the type of drug, a quantitative predictor, dose (X) will be used in a regression model for predicting Y. Let Z1=1 if drug is A, 0 otherwise, Z2=1 if drug is B, 0 otherwise, and Z3=1 if drug is C, 0 otherwise. what terms should be added to the model to create a model with interactions