One key assumption of classical linear regression model with OLS estimator is Cov (ui,uj) = O a. Briefly describe the assumption (50 words). b. Nominate an example of how a violation of this assumption might arise (you should specifically outline: what is the model in this example; what are the data used in this model; what is the purpose of this model; what are the reasons contributing to the violation in this model?) (200 words)