Sentence:  `` To be quite alone where there are no other human beings <WSD>is</WSD> sharply exhilarating ; it is as though some pressure had suddenly been lifted , allowing an intense awareness & & & a sharpening of the senses '' . 
Meaning of the word marked with <WSD> tags:  have the quality of being; (copula, used with an adjective or a predicate noun)