Why they used (stating) not( state)...althought they was talking about fact?
, the most comprehensive effort at measuring public opinion across more than 100 countries, the proportion of respondents (stating) that “having a democratic political system” is a “bad” or “very bad” way to run the country,

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Because the verb in the quote from the respondents is in the infinitive, so the verb "state" is in the infinitive also. Sorry, English sucks.

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