In terms of photosynthesis, C3 plants use C3 photosynthesis which uses CO2 in a 3-carbon compound and C4 plants use C4 photosynthesis which incorporates the CO2 in a 4-carbon compound. These compounds represent different types of sugar. It is difficult to tell which plant will be C3 and which will be C4. But, one fact is that C4 plants grow in arid regions mainly, so we can roughly say that the plants growing in dry regions under scarcity of water are C4 plants.