Which of the following best describes the total amount of carbon in the system made up of the Earth and its atmosphere?
A. It remains constant because oxygen converts to carbon in the geosphere and hydrosphere.
B. It continually increases as carbon is produced during cellular respiration and trapped in the atmosphere.
C. It continually decreases as carbon is used up during photosynthesis and trapped in the biosphere.
D. It remains constant because carbon cycles through the atmosphere, hydrosphere, biosphere, and geosphere.