Growth is limited for developed countries like the U.S.because _____.

A) they have to introduce new knowledge and technology
B) they have a unionized labor force
C) they tend to protect their industries by import substitution policy,which makes the domestic producers inefficient
D) the contribution of the agriculture sector is larger in their national output
E) they do not believe in free markets

Developed countries like the U.S. have already utilized most of their available resources, and their labor force is highly skilled and educated. Therefore, for further growth, they need to introduce new knowledge and technology, which is often costly and time-consuming. Additionally, protecting industries by import substitution policy and having a larger contribution of the agriculture sector in national output are not factors that limit growth. The unionized labor force and not believing in free markets can have their own constraints, but they are not the main reasons for limited growth in developed countries like the U.S.