Consider demand for physicians' services. In a two$-$ to three$-$page paper, develop a simple econometric model for the demand for physicians' services in the United States. Define the dependent variable as the annual spending on physicians' services, and assume it is normally distributed. Choose three independent variables that influence how much a family spends on physicians' services annually. Explain why you chose those varlables. Define in general terms the vector for the independent variable and the matrix of independent variables, assuming the rows are the data from individual cities and the columns variables.