Sentence:  One doctor made a careful survey of his patients and the reasons for their troubles , and he reported that 40 % of them worried about things that never happened ; 30 % of them worried about past happenings which were completely beyond their control ; 12 % of them worried about their health , although their ailments were imaginary ; 10 % of them worried about their friends , neighbors , and relatives , most of whom <WSD>were</WSD> quite capable of taking care of themselves . 
Meaning of the word marked with <WSD> tags:  have the quality of being; (copula, used with an adjective or a predicate noun)