Why might developing countries have only primary and secondary economic activities

A developing country might only have primary and secondary economic activities because the economy is not developed enough to engage in other levels of economic activities. Primary activities are the extraction or cultivation of natural resources and secondary activities are the manufacturing of finished goods. A developing economy may have only reached development of these sectors because of inefficiencies and weaknesses in the economy that it has not overcome through development yet. 

Ithink b is the right answer