A population of 1,000 healthy, at risk people is monitored for one year starting on January 1st and the development of cases of chicken pox is noted. No one has chicken pox at the start of the investigation. Twenty people develop chicken pox on June 30th and forty people develop chicken pox on September 30th. Twenty-four people were lost to follow-up on March 31st and twenty-four people were lost to follow-up on November 30th. None of those lost to follow-up had developed chicken pox prior to becoming lost. Assume that you can get chicken pox only once. What is the cumulative incidence of chicken pox in this population during the one-year period from January 1st through December 31st?
a. 20/1,000
b. 40/1,000
c. 60/1,000
d. 60/108