The muscle cell fluid of an athlete was tested immediately after a 1 O m race and was found to contain a high…

Analyze the question: After a short, intense race (the "1 O m" is likely a typo for 100 m), the athlete's muscle cells contain a lot of lactic acid. We need the cause.

Concept: Muscles normally respire aerobically, using oxygen to break down glucose completely:

Glucose + Oxygen → Carbon dioxide + Water + Large amount of energy

During a sprint, muscles need energy very quickly. Oxygen cannot be delivered fast enough, so the muscle cells break glucose down without oxygen (anaerobic respiration):

Glucose → Lactic acid + Small amount of energy

The lactic acid builds up in the muscle cells. This build-up causes cramps and muscle fatigue. After the race, the athlete breathes heavily to take in extra oxygen and pay back the "oxygen debt", which breaks down the lactic acid.

Solution (checking each option):

• A. Eating lactic acid would not put it into muscle cells in high concentration. Digestion and absorption don't work that way, so this is wrong.
• B. Injecting lactic acid is not a natural or reasonable explanation, so this is wrong.
• C. Anaerobic respiration in the muscles during the race produces lactic acid. This is correct.
• D. Lactic acid is not something we inhale from the environment, so this is wrong.

Answer: The athlete's muscles must have carried out anaerobic respiration during the race, which is option C.

Tip: Remember the pairing: animals/muscles → lactic acid, while yeast/plants → ethanol + carbon dioxide. Whenever you see "sprint," "intense exercise," "cramps," or "oxygen debt" in an exam, think anaerobic respiration and lactic acid.