Explanation: Transformers rely on the principle of electromagnetic induction (Faraday's Law) to operate. This requires a *changing* magnetic field through the primary coil to induce a voltage in the secondary. Direct current (DC), once steady, produces a constant magnetic field. Without a changing magnetic flux, no voltage is continuously induced in the secondary winding. Therefore, in general, a transformer cannot be used with a steady DC source (Option A). Applying DC continuously to a transformer can also lead to saturation of the core and overheating of the primary winding due to its low DC resistance. However, if the DC current is rapidly switched on and off, or periodically interrupted (pulsed DC), it *does* create a changing magnetic field. This fluctuating field can induce a voltage in the secondary winding, allowing the transformer to function. This principle is utilized in circuits like DC-DC converters, which chop DC into AC-like pulses to change voltage levels. Since both scenarios accurately describe how a transformer interacts with DC, statement A (general case) and statement B (specific exception) are both correct, making C the best answer.