During a routine check up,a 67-year-old man is found to have a level of serum alkaline phosphatase three times the upper limit of normal.Serum calcium and phosphorus concentrations and liver function test results are normal.He is asymptomatic.The most likely diagnosis is:

• A. Metastatic bone disease
• B. Primary hyperparathyroidism
• C. Paget's disease of bone
• D. Osteomalacia