Which of the following statements is true? Select one: O a. Endogenous and exogenous variables are both flow variables. Furthermore, endogenous variables influence the exogenous variable in a model. O b. Changes to endogenous variables can never be caused by shocks to a system. O c. In the model of a classical economy, government expenditure, is an exogenous variable. O d. Endogenous variables are flow variables, while exogenous variables are stocks. 1200 1 22-5- 27