Sentence:  Very small concentrations of these hydrides should be detectable ; in interstellar gas , concentrations as low as * * f molecules / * * f may <WSD>be</WSD> sufficient , as compared to the * * f hydrogen atoms * * f required for detection of the 21 - cm line . 
Meaning of the word marked with <WSD> tags:  have the quality of being; (copula, used with an adjective or a predicate noun)