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(A) Glucose
(B) Triglyceride
(C) Glycogen
(D) Proteins
Answer: (A) Glucose
Explanation: Skeletal muscle cells primarily use glucose as their main energy source during high-intensity activities. While glycogen (stored glucose) is also crucial, glucose is the primary substrate utilized for rapid energy production. Triglycerides and proteins are less immediate sources of energy for muscles, as triglycerides are used for long-term energy storage and proteins are mainly used for repair and growth.