During vigorous exercise, muscles respire anaerobically because

  1. they have no glucose
  2. they have too much oxygen
  3. lactic acid is needed
  4. not enough oxygen reaches them
Answer

D) not enough oxygen reaches them

Oxygen supply cannot meet demand. O Level Biology · Topic 10 Respiration · 10.3 Anaerobic respiration

Biology MCQ · More O Level Biology: Respiration MCQs · Practise