The following are the quarterly petrol sales (in thousands of litres) of a local petroleum company for the past three years. Year Quarter 2005 2006 2007 1 18 41 54 2 56 69 62 3 82 89 90 4 27 66 66100 Sales (in thousand litres) 80 60 40 20 Quarter petrol sales 0 Qtr 1 2005r 2, 2005r 3, 20Qar 4, 20Qar 1. 20ear 2, 20ear 3, 2008r 4, 200Ar 1, Time periods 2007tr 2, 2007r 3, 2007tr 4, 2007(ii) Compute the four-quarter centred moving average for the time series given. [4 marks] Interpret the result obtained. [2 mark] (iii) Compute and interpret the petrol sales seasonal index for each quarter [5 marks]