Question 7 The following plots have been obtained for a time series. a) Suggest an appropriate ARIMA model based on the below plots. (2 marks). 120- 90 60 30 - 1980 Jan 1990 Jan 2000 Jan 2010 Jan 2020 Jan Month 0.2 0.2- 0.1 -- 0.1 -- pact act 0.1 - - - - - -0.1 - - . 12 18 24 12 18 24 lag [1M] lag [1M] b) The following ARIMA output has been obtained from R. Based on this output, explain which model would you recommend for forecasting? (2 marks). # # .model sigma2 log_lik AIC AICC BIC ar_roots ma roots # ## 1 arima011 17.1 -1393. 2787. 2787. 2806. ## 2 arimal10 17.1 -1399. 2780. 2780. 2815. ## 3 auto 17.4 -1392. 2788. 2788. 2807. c) If your selected model in part b) above has a p-value of 0.900 in the Ljung-Box test, would you recommend using this model? Explain why or why not. (2 marks)