Which of the following statements is the most accurate characteristic of cohort studies?

A) It is an efficient study designs for studying rare disease.
B) The temporality of the exposure preceding the outcome is hard to determine.
C) The incidence rate of disease in the exposure group is compared directly to the incidence rate of disease in the unexposed group.
D) The exposure and outcomes are captured at the same time.
E) The participants are randomly allocated to exposure and nonexposure categories.

The incidence rate of disease in the exposure group is compared directly to the incidence rate of disease in the unexposed group because direct comparison of incidence rate,the risk,is one of the most distinctive characteristics of cohort study."It is an efficient study designs for studying rare disease" is not correct.A cohort study is not an efficient study design for studying a rare disease."The temporality of the exposure preceding the outcome is hard to determine" is not correct.The temporarily of the exposure preceding the outcome is easy to determine."The exposure and outcomes are captured at the same time" is not correct.In a cohort study,exposure is measured at the beginning of the study,then outcomes are captured."The participants are randomly allocated to exposure and nonexposure categories" is not correct.In a cohort study,investigators cannot allocate the exposure status; this is an explanation of randomized clinical trial.