A production possibilities frontier is a straight line when (　　)
选项：

A: the more resources the economy uses to produce one good, the fewer resources it has available to produce the other good.
B: an economy is interdependent and engaged in trade instead of self-sufficient.
C: the rate of tradeoff between the two goods being produced is constant.
D: the rate of tradeoff between the two goods being produced depends on how much of each good is being produced.