A small town has 1,000 people, 600 of whom are healthy and 400 of whom are sick. The annual expected medical claims of the healthy and sick are $500 and $9,000, respectively. If the insurance company cannot determine who is healthy and who is sick prior to the purchase of insurance, the annual premium must be at least $____________.

a. 3,900
b. 9,000
c. 2,000
d. 4,750