FILE Hybrid cars have gained popularity because of their fuel economy and the uncertainty regarding the price of gasoline.

All automakers, including the Ford Motor Co., have planned to significantly expand their hybrid vehicle lineup (CNN.com, November 9,2005).The following table contains quarterly sales of Ford and Mercury hybrid cars. The data are also on the text website, labeled Sales Data.

Year Q1 Q2 Q3 Q4 2006 6,192 5,663 4,626 5,645 2007 5,149 6,272 5,196 6,545 2008 5,467 5,235 3,160 7,007 2009 5,337

a. Plot the above series. Comment on the trend and seasonal variation in the sales of hybrid cars.

b. Compute and interpret the seasonal indices.