9. For a yellow-orange gas X, which will partially undergo trimerization to give a colorless gas $(X_{3})$ at $25^{\circ}C$.

3 X (yellow gas) → X₃ (colorless gas)

 If it was found that as the reaction temperature increases, the color of the two gaseous mixtures is getting darker. Which statement is correct?
(A) The trimerization reaction is an endothermic and also exothermic reaction.
(B) Cannot determine it is endothermic reaction or exothermic reaction.
(C) The trimerization reaction is an exothermic reaction.
(D) The trimerization reaction is an endothermic reaction.
(E) All is correct.