Question: what is the meaning of b.c. and a.d
[{'answer': 'anno Domini', 'provenance': [{'wikipedia_id': '1400', 'title': 'Anno Domini', 'start_paragraph_id': 1, 'start_character': 279, 'end_paragraph_id': 1, 'end_character': 290, 'bleu_score': 1.0, 'section': 'Section::::Abstract.'}]}, {'answer': 'before Christ', 'provenance': [{'wikipedia_id': '1400', 'title': 'Anno Domini', 'start_paragraph_id': 1, 'start_character': 19, 'end_paragraph_id': 1, 'end_character': 32, 'bleu_score': 1.0, 'section': 'Section::::Abstract.'}]}, {'provenance': [{'wikipedia_id': '1400', 'title': 'Anno Domini', 'start_paragraph_id': 1, 'start_character': 10, 'end_paragraph_id': 1, 'end_character': 290, 'bleu_score': 0.678436523498337, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 468346310018995864}, 'section': 'Section::::Abstract.'}]}]