Explanation: A dynamotor is a specialized rotary converter that combines a DC motor and a DC generator into a single unit, often utilizing a common armature core and a common field winding. This integrated design is the basis for statement A, making it true. Due to their compact size and ability to convert low DC voltage (like from a vehicle battery) to much higher DC voltage required by vacuum tube circuits, dynamotors were extensively used in mobile, airborne, and portable radio equipment. This confirms statement B, as they were predominantly battery-powered for this voltage conversion. The shared components (field winding, sometimes armature core) and integrated construction generally result in a slightly higher efficiency, lighter weight, and smaller footprint compared to separate motor-generator sets performing the same function. Therefore, statement C is also true. Since all three statements accurately describe characteristics of a dynamotor, "All of the above" is the correct choice.