February 2nd is World Wetlands Day. The slogan of this year's World Wetlands Day is "Join hands to protect wetlands and cope with climate change". Wetland species resources are rich and widely distributed. According to "the process of photosynthesis can be expressed as CO2+H2O irradiating chloroplast starch +O2" and the conservation of elements according to the law of mass conservation, we can know that the types of elements are unchanged before and after the reaction, so we know that starch must contain carbon, hydrogen and possibly oxygen.

So it should be: carbon, hydrogen and oxygen.