Final answer:
Horizontal vent pipes shall be level or graded to drip back by gravity to the roof. Option b.
Step-by-step explanation:
Horizontal vent pipes shall be level or so graded as to drip back by gravity to the roof. This ensures that any condensation or moisture that accumulates in the vent pipes will flow back towards the roof and not accumulate in the pipes. Proper grading of the vent pipes helps prevent blockages and promotes the efficient flow of air and gases in the plumbing system.
Horizontal vent pipes in plumbing systems should be installed to ensure they remain either level or are graded to allow any condensation or moisture that collects within them to drip back by gravity to the source of the drainage system. The correct answer to which horizontal vent pipes should drip back by gravity is the drainage system, which is not explicitly listed in the options provided. However, if we need to choose the best option from the ones given, we would tend to opt for the vent pipes to drip back towards the basement, since this is commonly where the house's drainage system would begin or be routed.