Sentence:  Theoretically , they <WSD>are</WSD> capable of extracting their required oxygen either from the surrounding air ( Ghoreyeb and Karsner , ' 13 ) or from pulmonary arterial blood ( Comroe , ' 58 ) . 
Meaning of the word marked with <WSD> tags:  have the quality of being; (copula, used with an adjective or a predicate noun)