According to the Uniform Plumbing Code, back siphonige is the flowing back of used, contaminated, or poluted water from a plumbing fixture or vessel into a water supply pipe due to a ____ in such pipe.

According to the Uniform Plumbing Code, back siphonige is the flowing back of used, contaminated, or poluted water from a plumbing fixture or vessel into a water supply pipe due to a ____ in such pipe.