Why are the stubs on a compressor and filter driers more difficult to braze than copper pipe?

The correct answer points to the fact that stubs on a compressor and filter driers have a thin copper coating that is particularly susceptible to oxidation when exposed to excessive heat during the brazing process. This oxidizing effect occurs because when the copper coating is overheated, it can create a layer of oxide that impedes proper adhesion and flow of the brazing material. Additionally, the oxidation can weaken the integrity of the joint, leading to potential leaks in the refrigeration system.

While other options might suggest various physical attributes or design factors, they do not capture the specific challenge posed by the coating's reaction to heat. For instance, the suggestion that stubs are thicker than copper pipes does not accurately reflect how thickness directly impacts the brazing process itself; rather, it is more the material composition and its behavior at high temperatures that is critical in this scenario.