polish my answer for item (a); keep it concise in this format :The key assumption is the The residuals are independently and identically distributed (i.i.d.) 1) The errors are independent of each other; 2: the errors have a common variance; 3) the errors are normally distributed with a mean of 0 and constant variance given X. The normality of residuals is crucial because the t-statistic derived from non-normally distributed residuals may not follow a t-distribution, impacting the validity of hypothesis tests and confidence intervals