Sentence:  Thus , although the agenda of external assistance in the economic sphere are cumulative , and many of the policies suggested for nations in the earlier stages remain relevant , the basic purpose of American economic policy during the later stages of development should be to assure that movement into a stage of self-sustaining growth is not prevented by <WSD>lack</WSD> of foreign exchange . 
Meaning of the word marked with <WSD> tags:  the state of needing something that is absent or unavailable