If a tube bursts or cracks, it is generally the result of ________, ________, or ________.

The correct answer highlights the primary reasons that can lead to a tube bursting or cracking. Excessive vibration can cause a physical strain on the material of the tube, leading to fracture points over time. When tubes are improperly installed, they may experience undue pressure at connections or bends, which weakens their structural integrity and increases the risk of failure. Similarly, damage from external sources can compromise the tube's material, making it more susceptible to bursting.

In this context, these factors directly relate to how the tube is handled, installed, and subjected to conditions that exceed its designed limitations. Understanding these variables aids in recognizing preventative measures that can be implemented during tube installation and maintenance to avert incidents of failure.