The level of oxygen in the air is measured in a garden and on a city road. Which location has a higher level of oxygen and why? A. The oxygen level is the same in both places because oxygen production by photosynthesis remains constant, regardless of location. B. The oxygen level in the city is higher because the exhaust gases from vehicles produce oxygen through the release of carbon dioxide. C. The oxygen level in the garden is higher because the plants are constantly releasing oxygen through photosynthesis.