Final answer:
Doug feels his climb to Camp 3 is over due to fatigue from the harsh conditions of Mount Everest, such as extreme cold and low oxygen levels.
Step-by-step explanation:
During the ascent to Camp 3, Doug feels that his climb is over because b) He becomes too fatigued to continue climbing. The harsh conditions of climbing Mount Everest, such as extreme cold, low oxygen levels, and low barometric pressure, contribute to climbers feeling exhausted and potentially jeopardizing their ability to continue the ascent. In literature detailing such experiences, fatigue is often portrayed as an overwhelming force that makes further progress seem impossible, hence why Doug might feel his climb is over.