Final answer:
Chemical Y, with a residence time of 3.5 days, poses the greatest risk of exposure to the nearby community over the span of a week following a spill, since the other chemicals clear more rapidly.
Step-by-step explanation:
When evaluating the exposure risk to the nearby community from a spill containing carcinogens, it is crucial to consider the residence time of these chemicals in the environment. The carcinogens in the question - Chemical X, Chemical Y, and Chemical Z - have different residence times: 2.8 days, 3.5 days, and 17.2 hours respectively.
Given that all three carcinogens are equally harmful at equal concentrations, the one with the shortest residence time will pose the least long-term risk because it will be present in the environment for a shorter duration. Since a week has 168 hours, and Chemical Z, with a residence time of 17.2 hours, will be cleared much faster than the other two chemicals, Chemical Y, with a residence time of 3.5 days, possesses the greatest risk of exposure over the course of a week following the spill.