The following scatterplot shows world population (millions) plotted against 5 year intervals from 1950 through 2000.
a. Based on the scatterplot, is the linear model appropriate? Explain. 
b. A linear regression model was estimated to describe the trend in world population over time. Based on the plot of residuals versus predicted values shown below, would you consider this model appropriate? Explain. 
c. Would a re-expression of the data help? If so, what would you recommend?
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