Can't seem to figure this one out.

A screening program is initiated for detection of vaginal C. trachomatis infection among first year college women students. At the initial screening Chlamydiae is found in 500 of the 2500 students. One year later, the screening shows that the infection has affected an additional 200 students. Which of the following is the annual incidence of C. trachomatis infection in this population of women students.

A.) 8%
B) 10
C) 16
D) 20
E) 28

Incidence is all new cases over the total population. So I did 200 over 2500 and got 8% but it was wrong, I don't have the correct ones, does anyone know how to solve it?

As 2500 is 100%, then 200 is X= 8%....annual incidence is 8%.

I put 8% but that was marked wrong, so it can't be that.

Incidence = New cases / Population at risk.
500 were already positive from the previous year's screening.
Therefore, population at risk = 2500 - 500 = 2000
Annual incidence = 200/2000 = 1/10 = 10%

Dear friend

I pick 10% for the same reason.

Best of luck