A study collected data from medical records for 1000 patients aged 70-90 years old and found that 250 patients had Alzheimer's disease.

A study collected data from medical records for 1000 patients aged 70-90 years old and found that 250 patients had Alzheimer's disease. 100 of these patients with Alzheimer's disease were diagnosed within the past year and 100 of them died during the past year. What is the prevalence of Alzheimer's disease at the end of this study? (Assume no one was cured of the disease.)

(a) 10%

(b) 17%

(c) 15%

(d) 25%

(e) 27%