State with brief reason whether the following statements are true, false, or uncertain:

a. In the presence of heteroscedasticity OLS estimators are biased as well as inefficient.

b. If heteroscedasticity is present, the conventional t and F tests are invalid.

c. In the presence of heteroscedasticity the usual OLS method always overestimates the standard errors of estimators.

d. If residuals estimated from an OLS regression exhibit a systematic pattern, it means heteroscedasticity is present in the data.

e. There is no general test of heteroscedasticity that is free of any assumption about which variable the error term is correlated with.

f. If a regression model is mis-specified (e.g., an important variable is omitted), the OLS residuals will show a distinct pattern.

g. If a regressor that has nonconstant variance is (incorrectly) omitted from a model, the (OLS) residuals will be heteroscedastic.