Sentence:  For an experiment to qualify as a binomial experiment , it must have four properties : ( 1 ) there must be a fixed number of trials , ( 2 ) each trial must result in a `` success '' or a `` failure '' ( a binomial trial ) , ( 3 ) all trials must have identical probabilities of success , ( 4 ) the trials must <WSD>be</WSD> independent of each other . 
Meaning of the word marked with <WSD> tags:  have the quality of being; (copula, used with an adjective or a predicate noun)