Which of the following is true of an economy\textquotesingle s production possibilities curve? 
选项为:A:It shows the combinations of any two resources that can be used to produce an efficient level of output.  B:It shows the alternative combinations of goods that can be produced by fully employing scarce resources.  C:It must be a straight line when all resources are fully employed.  D:It is bowed in (convex to the origin) because of changing levels of technology.  E:It is bowed out (concave to the origin) when marginal opportunity costs are constant.  请回答其中一个选项.