Why are liners laid down at the bottom of a landfill before waste is deposited in the landfill?

A.) to prevent bacteria from the soil entering and decomposing the trash
B.) to provide a system for capturing usable water from trash
C.) to prevent hazardous substances in the trash from leaking into the groundwater
D.) to provide a way to collect methane gas building up in the trash

If you could help me...That would be great. Thank you ;) :)