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Through the chemical equation X+3H 2? High temperature? . ? W+3H 2 O, after the reaction, W and 3H 2 O contain 1 W atoms, 6 H atoms and 3 O atoms, while 3H 2 only contains 6 H atoms before the reaction, so X molecule is composed of 1 W atoms and 3 O atoms, and the chemical formula of substance X is WO 3；;

A, the chemical equation of the reaction WO 3 +3H 2? High temperature? . ? W+3H 2 O, the reaction is a substitution reaction between simple substances and compounds to form simple substances and compounds, not a combination reaction, so option A is incorrect;

B, according to the algebraic sum of valence of each element in the compound is 0, it can be known that the valence of W element in WO 3 before the reaction is +6, while that in simple substance W after the reaction is 0, and the valence of W element before and after the reaction changes from +6 to 0, so option B is incorrect;

C, by the chemical equation X+3H 2? High temperature? . ? W+3H 2 O, the chemical formula of substance X is WO3；; So option c is correct;

D. Before the reaction, the valence of H element in H 2 was 0, after the reaction, the valence of H element in H 2 O was+1, and the valence of hydrogen in the reaction changed from 0 to+1. Option D was incorrect;

So choose C.