Point prevalence rate is defined as the number of ()
选项：

A:new cases in a time period
divided by the average duration of the disease
B:new cases in a time period
divided by the population at the midpoint of the disease
C:existing cases in a ime
period divided by the population at the beginning of the time period
D:existing cases at a point of
time divided by the population at that point of time
E:existing cases at a point of
time divided by the population at the end of the time period