Passage Summary: Cleaning supplies in landfills turn into toxic gas when they rot, so making parks out of those landfills is making people sick.

Conclusion: Converting landfills into public parks is currently causing harm to human health.

Reasoning: Landfills contain cleaning products that, when broken down by bacteria, release toxic vapors.

Analysis: The argument jumps from the existence of toxic vapors underground to the conclusion that people using the parks above are being harmed. This requires a 'Necessary Assumption' that links the toxins to the people. Specifically, the argument assumes that these vapors are not safely contained and actually reach the park-goers in harmful amounts. To find the right answer, look for a statement that, if false, would completely break the link between the buried trash and the health of the people walking on top of it.