【跪求答案】【祈求高人指点】几道计量难题！

各位论坛大侠，各位学长学姐，各位老师前辈，您好！

遇到4道计量经济课程方面的题目，不太会解答！

跪求高人指点，提供答案！

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再次跪谢，感恩！

第一题

The Gauss-Markov theorem proves that the OLS estimators are unbiased and efficient under some conditions.

a) Explain how the heteroscedasticity affects the unbiasedness and the efficiency of the OLS estimator.

b) Explain how the multicollinearity affects the unbiasedness and the efficiency of the OLS estimator.

第二题

Suppose that X2 and X3 are highly correlated in the following multiple regression.

第三题

Consider the following regression which includes an interaction term.

第四题

Consider the following true model which includes two significant factors, X1 and X2.