What is a key reason why electric drills should not be used near flammable materials?

Using electric drills near flammable materials poses a significant risk primarily due to the potential for sparks generated by the electric motor. Electric motors, especially when they are under load or malfunctioning, can create sparks that may ignite nearby combustible substances. This risk is particularly heightened in environments where flammable vapors, liquids, or dust are present.

While the other options address some safety concerns, such as overheating leading to equipment failure or the risk of electric shocks, the generation of sparks is the most critical factor when it comes to fire hazards in areas with flammable materials. Ensuring that tools like electric drills are used in safe environments away from potential ignition sources is a fundamental practice in maintaining safety during aircraft metal structural repairs and related tasks.