The Morbidity and Mortality Weekly Report published by the U.S. Centers for Disease Control is an example of:

(a) active surveillance

(b) passive surveillance

(c) retrospective cohort surveillance

(d) cross-sectional survey surveillance

I believe it is passive surveillance (b) because the state and local health departments provide information to the CDC. Because they're just reporting it to the CDC, that would be passive. I've never heard of (c) or (d). Are they even real?