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遇到4道计量经济课程方面的题目，不太会解答！

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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.