It may be carbon dioxide produced by carbon monoxide and copper oxide (reasonable); ④ carbon monoxide

Analysis of test questions: ① Black copper oxide turns red, which means that A undergoes redox reaction, and copper oxide loses oxygen and undergoes reduction reaction; Only CO is reducible among CO, CO 2 and H 2 O, so the original COlorless gas must contain co.

(2) Anhydrous copper sulfate turns blue when it meets water, and now colorless gas must contain no water vapor, so anhydrous copper sulfate will not change color;

(3) Clarify the turbidity of limewater, indicating that the gas flowing into C contains carbon dioxide, but the carbon dioxide here may be the original carbon dioxide or carbon monoxide and copper oxide, so it is not certain that there is carbon dioxide in the original colorless gas. Because the original COlorless gas must contain co, carbon monoxide can not fully react, and if it is directly discharged into the air, it will pollute the air, so it is collected by balloon.