Sentence:  Although modern scholars have expressed surprise that `` the simple magic square of three '' , a mere `` mathematical puzzle '' , <WSD>was</WSD> able to exert a considerable influence on the minds and imaginations of the cultured Chinese for so many centuries , they could have found most of the answers right within the square itself . 
Meaning of the word marked with <WSD> tags:  have the quality of being; (copula, used with an adjective or a predicate noun)