Read the passage. Oxygen is one of the most essential elements found on Earth. Humans and all living things depend on oxygen for survival. This vital element is obtained different ways. The primary source for oxygen is from the ocean, which gives more than fifty percent of Earth's oxygen and most come from tiny ocean surface plants called phytoplankton. In the process of photosynthesis, phytoplankton releases oxygen into the water. Photosynthesis is also the process used on land by plants, grass, and trees to convert the sun's energy and nutrients to be released as oxygen. Other forms of oxygen are in the Earth's crust and minerals, and do not contribute to the oxygen we breathe in the atmosphere. What does the author imply in the passage? A. Oxygen cannot be produced without plants and trees. B. Oxygen found in the Earth's crust is important to plant life. C. There would be enough oxygen in the air without phytoplankton. D. The only source of oxygen in the atmosphere is from photosynthesis.