Statistics: Cohort or case control

The case is as followed (and sorry for the bad translation): "There is much that indicate that exposure to different chemicals can increase the risk for renal cancer. Would you use a cohort study or a case control study to answer the question?" There is only one best answer. I hope that some of you can explain to me how this is so obvious. For me it's not.
Source: Student Doctor Network - Category: Universities & Medical Training Authors: Source Type: forums