Final answer:
The closest possible answer to the student's question based on the EPA's action level for lead in drinking water is '3 times greater; 15 ppb', considering the EPA's action limit is 15 ppb as mentioned in the examples provided.
Step-by-step explanation:
The Environmental Protection Agency (EPA) sets the action limit for lead in drinking water at 15 parts per billion (ppb). According to one of the exercises you've provided, the EPA has identified the maximum allowable concentration of lead in drinking water to be 9.0 ppb. However, these measurements can be subject to different interpretations or outdated guidelines, hence different numbers might be referenced, like 15 ppb as the action limit in other documents. When an EP detects lead levels at considerable multiples of the EPA limit, it indicates a significant worry regarding the safety of drinking water. Looking at the available choice options, none explicitly matches the figures in the provided information, which might suggest an error or a difference in the contextual source. The closest possible answer based on the EPA's action level mentioned in your references would be 3 times greater than 15 ppb (c. 3 times greater; 15 ppb) if we use the correct EPA action limit provided in the example.