Sentence:  So be it - then we must embark on a crash program for 200 - megaton bombs of the common or <WSD>hydrogen</WSD> variety , and neutron bombs , which do not exist but are said to be the coming thing . 
Meaning of the word marked with <WSD> tags:  a nonmetallic univalent element that is normally a colorless and odorless highly flammable diatomic gas; the simplest and lightest and most abundant element in the universe