Used car dealerships generally have sales quotas that they strive to hit each month, quarter, and calendar year. Consequently, buying a used car toward the end of those periods presents a great opportunity to get a good deal on the car. A local dealership has compiled monthly sales data for used cars (Cars) from 2014-2019, a portion of which is shown in the accompanying table. 

a. Estimate the linear regression models using seasonal dummy variables with and without the trend term. 

b. Determine the preferred model and use it to forecast used cars sales in the first two months of 2020.

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Date Jan-2014 Feb-2014 I Dec-2019 Cars 138 179 ⠀ 195