The carbon is among the most abundant element found in the living organisms. The carbon enters the food web through the producers via the process by which they make their food, which is known as photosynthesis. The plants during the process of photosynthesis, take in carbondioxide, water and capture the sunlight to make ATP. The carbon from all these elements enters the producers and then is passed on from one trophic level to another.
Hence, the blanks can be filled with 'producers, and photosynthesis'.
