Sentence:  A more tenable explanation for the change in reactions <WSD>is</WSD> that the added knowledge and increased familiarity with the total situation made it possible for these subjects to be less guarded and to relax , since any reaction seemed acceptable to the examiner as `` normal '' . 
Meaning of the word marked with <WSD> tags:  have the quality of being; (copula, used with an adjective or a predicate noun)