In a county that does not require measles vaccination, a preschool experienced a prolonged measles outbreak among its students beginning in September 2016 through December 2016.

To calculate the cumulative incidence or risk of measles among students at the preschool, which denominator should you use?

The number of susceptible students at the end of the period (December 2016)

The number of susceptible students at the midpoint of the period

The number of susceptible students at the beginning of the period (September 2016)

The average number of susceptible students during the outbreak