Solution:

• A drain trap or gully is used to connect the pipe from your home into an underground drainage system.
• The aim of the drain trap is to allow waste to go down the drainage system, whilst preventing harmful insects, vermin and sewer gases from entering the property by going back up the pipe.
• Thus, in plumbing, a trap is a device shaped with a bending pipe path to retain fluid to prevent sewer gases from entering buildings while allowing waste to pass through.
• In oil refineries, traps are used to prevent hydrocarbons and other dangerous gases from escaping through drains.