After 5 days of hospitalization, the client who is receiving morphine sulfate for pain control asks for more pain medication with increasing frequency and exhibits increased anxiety and restlessness. His physical condition is stable. What is the probable cause of his behavior（）
A. His morphine dosage is too high.
B. His coping mechanisms are exhausted.
C. He is becoming addicted to the narcotic.
D. He has developed tolerance to his narcotic dosage.
