E-commerce. Quarterly e-commerce retail sales (in millions of dollars) in the United States are provided.

(Source: www.census.gov.) Use this time series to answer the following questions.

a) Fit a linear trend model to this series but do not use the last two quarters (Q4 2007 and Q1 2008).

b) Fit an exponential trend model to this series but do not use the last two quarters.

c) Use both models to forecast the quarterly values for Q4 2007 and Q1 2008. Which model produces better forecasts?

d) What other time series components (besides trend) are likely present in this series?