Surface of the copper vessels and utensils turns black with time. It is the result of

a) Reaction of copper with the Nitrogen
b) Reaction of moisture with the copper
c) Oxidation of the copper to copper oxide
d) Due to presence of carbon dioxide in the air

Ans-[C] 

Explanation-This is due to the formation of copper oxide in the presence of oxygen in the air. Copper is said to be oxidized in this way.