Question: which device is used to convert ac to dc
[{'answer': 'Rectifier', 'provenance': [{'wikipedia_id': '618120', 'title': 'Electric power conversion', 'start_paragraph_id': 21, 'start_character': 12, 'end_paragraph_id': 21, 'end_character': 21, 'bleu_score': 1.0, 'section': 'Section::::AC power conversion.:AC to DC.\n'}]}, {'answer': 'Switched - mode power supply'}, {'answer': 'Mains power supply unit ( PSU )'}]