A serious safety accident occurred in a certain coal mine. Regarding the cause of the accident, the four responsible persons of A. B, C, and D concluded that: A: If the direct cause of the accident was equipment failure, then someone must have violated the operating rules. B: Someone did violate the operating regulations, but the direct cause of the accident was not a failure of the equipment. C: The direct cause of the accident was indeed a failure of the device, but no one violated the operation. D: The direct cause of the accident was the failure of the equipment. If only one of the above assertions is true, which of the following assertions is true? ["A's assertion is true, someone violated the operating rules.", "A's assertion is true, but no one violates the operating procedures.", "B's assertion is true.", "C's assertion is true."]