The inference that can be made is : For these plants, rates of photosynthesis at CO2 concentrations of 1000 ppm and 750 ppm are very similar.
How is the CO2 consumption similar
The similarity can be seen from the fact that from 750 ppm for both plants the CO 2 consumption became constant for both
They consumed CO2 at different levels, but these levels were on a constant rate from 750 to 1000
Hence we conclude that The inference that can be made is : For these plants, rates of photosynthesis at CO2 concentrations of 1000 ppm and 750 ppm are very similar.