During the light independent reaction, carbon dioxide is fixed by adding it to a

By adding it to a 5-carbon compound.  

In plants, carbon dioxide gets inside the leaf with the help of spores known as stomata and disperses into the stroma of the chloroplast, the location of the Calvin cycle reactions. These reactions are also known as light-independent reactions, as they are not directly mediated by light.  

In the Calvin cycle, the carbon atoms from carbon dioxide are fixed and are utilized to generate three carbon sugars. One of the stages of the Calvin cycle is carbon fixation. In carbon fixation, a molecule of carbon dioxide merges with a five-carbon acceptor molecule, RuBP (ribulose-1,5-bisphosphate), catalyzed by the enzyme rubisco.  

the answer is a or when it says crossing over

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