Why is forecasting GDP in the long run so difficult? a $)$ There is no regularity in the frequency and amplitude of fluctuations in real GDP$.$ b$)$ Structural and statistical forecasting provide differing results. c$)$ There is no expertise in forecasting the long$-$run trend in real GDP$.$ d$)$ These is an absence of data that provides any information on future trends. e$)$ Current models are not sophisticated enough to conduct this exercise.