Typically, the higher the level of income per person in a country, the higher the level of spending per person on health care. This relationship between income and spending indicates that health care is a

An inverted structure in futures commodity prices (prices falling over time) from the December to May contract

A. potentially rewards storing the commodity for selling later. B. means potentially higher expected profits from postponing selling to May. C. usually means higher expected profits from selling farm output in December rather than in May. D. Both A and B.

HAC standard errors should be used because

A) they are convenient simplifications of the heteroskedasticity-robust standard errors. B) conventional standard errors may result in misleading inference. C) they are easier to calculate than the heteroskedasticity-robust standard errors and yet still allow you to perform inference correctly. D) when there is a structural break, then conventional standard errors result in misleading inference.