sentences:  For "P" a "c"-ary predicate, formula_21 is true if and only if when formula_22 is interpreted as formula_23 formula_24 is true.A query in FO will then be to check if a first-order formula is true over a given structure representing the input to the problem.One should not confuse this kind of problem with checking if a quantified boolean formula is true, which is the definition of QBF, which is PSPACE-complete.