By how much must the main vent be higher than any vertical wall within a distance of 10 feet?

The correct answer indicates that the main vent must be at least 2 feet higher than any vertical wall within 10 feet. This requirement is essential for proper venting of gases and ensuring that any exhaust or fumes can safely disperse into the atmosphere without being obstructed by nearby structures. The height helps to prevent backdrafts, where gases could potentially flow back towards the appliance rather than safely vent away.

The regulation is grounded in safety standards designed to minimize the risk of gas accumulation, which can lead to hazardous situations. Adequate height clearance is crucial to ensure that performance and efficiency of the ventilation system are maintained, ultimately safeguarding the environment and health.

The other options suggest varying heights, but they do not meet the established safety standards for effective venting as adequately as the requirement of 2 feet does.