Sentence:  This was later known to <WSD>be</WSD> the result of concentrating the minute amount of complete antibody found in these sera ; when the insoluble fraction was suspended in a volume of saline equal to that of the original serum sample , no complete antibody activity could be detected . 
Meaning of the word marked with <WSD> tags:  have the quality of being; (copula, used with an adjective or a predicate noun)