Problem 1: US Real GDP The reference mode of the US GDP is shown in Figure 1 US Real GDP 8000 AveragrowtRat8.45%/year DoublingTime 20years 4000 0 1850 1900 1950 2000 Figure 1: US GDP - (Sterman, 2001) 1. Identify the variables that might be responsible for the growth of the US GDP (2 points) 2. Sketch a causal loop diagram to capture the behavior you identify (2.5 points) 3. Identify potential negative feedback that might halt growth in the system (0.5 point)