According to the statistical data of China's fiscal revenue Y and GDP X from 1978 to 2000, the following econometric model can be established.

(1) What is the autocorrelation of econometric models?

(2) Test whether there is first-order autocorrelation in this model. Why?

(3) What influence will autocorrelation have on the econometric model established?

(4) If there is autocorrelation in the model, try to write the methods and steps to eliminate the - order autocorrelation.

(Critical D_L= 1.24, D_U=1.43)