An analysis of production possibilities curves indicates that the reason why underdeveloped nations have difficulties increasing their economic growth rates is because:

a. low population growth rates mean fewer workers to produce food and other necessities.
b. their production possibilities curves shift in when resources are increased.
c. their production possibilities curves are positively sloped, unlike those in more developed economies.
d. they must cut back their already meager consumption levels to increase capital production.
e. the opportunity cost of shifting resources from consumption goods to capital goods is relatively low.