Explanation: The fundamental difference between conductors and nonconductors lies in the availability of free electrons. In a good conductor, such as metals used for wires and antennas in amateur radio, the atoms have outer electrons that are loosely bound and can move freely throughout the material. This "sea" of free electrons (A) allows for easy flow of electric current. Conversely, in a nonconductor (or insulator), electrons are tightly bound to their respective atoms and are not free to move easily. This results in a very small number of free electrons (B), making it difficult for current to flow. Therefore, options A and B accurately describe the key distinction, making D the correct choice. Option C is incorrect as it contradicts this essential difference in electron mobility.