What symptom would indicate a TRU failure on the DC bus, and what are common root causes?

What symptom would indicate a TRU failure on the DC bus, and what are common root causes?

• A. Symptom: erratic voltage on DC bus; root causes: lubrication issues.
• B. Symptom: loss of DC bus voltage or erratic voltage; root causes: failed rectifier diodes, transformer winding faults, or blown fuses.
• C. Symptom: overvoltage on DC bus; causes: reversed polarity of battery.
• D. Symptom: excessive hum; causes: loose connections.

Answer: (B)