Question: when did the flash first appear on arrow
[{'answer': 'second season', 'provenance': [{'wikipedia_id': '52475215', 'title': 'Flash vs. Arrow', 'start_paragraph_id': 2, 'start_character': 113, 'end_paragraph_id': 2, 'end_character': 126, 'bleu_score': 1.0, 'section': 'Section::::Abstract.'}]}, {'answer': "the eighth episode of Arrow 's second season"}, {'answer': "in the eighth episode of Arrow 's second season"}, {'provenance': [{'wikipedia_id': '52475215', 'title': 'Flash vs. Arrow', 'start_paragraph_id': 21, 'start_character': 0, 'end_paragraph_id': 21, 'end_character': 937, 'bleu_score': 0.921225998660634, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 5370671793401554535}, 'section': 'Section::::Production.:Development.\n'}]}, {'provenance': [{'wikipedia_id': '52475215', 'title': 'Flash vs. Arrow', 'start_paragraph_id': 2, 'start_character': 0, 'end_paragraph_id': 2, 'end_character': 492, 'bleu_score': 0.933831427081359, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 5370671793401554535}, 'section': 'Section::::Abstract.'}]}, {'provenance': [{'wikipedia_id': '52475215', 'title': 'Flash vs. Arrow', 'start_paragraph_id': 1, 'start_character': 0, 'end_paragraph_id': 1, 'end_character': 605, 'bleu_score': 0.9427836745142131, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 5370671793401554535}, 'section': 'Section::::Abstract.'}]}, {'provenance': [{'wikipedia_id': '52475215', 'title': 'Flash vs. Arrow', 'section': 'Section::::Abstract.', 'start_paragraph_id': 2, 'end_paragraph_id': 2, 'meta': {'evidence_span': ['A crossover between the two series was announced in July 2014, after Barry Allen had been introduced in "Arrow"s second season ahead of the debut of "The Flash"', 'July 2014, after Barry Allen had been introduced in "Arrow"s second season ahead of the debut of "The Flash". By the following September, the scripts for each', 'A crossover between the two series was announced in July 2014, after Barry Allen had been introduced in "Arrow"s second season ahead of the debut of "The Flash". By the following September, the scripts for each episode had been written, with filming occurring shortly before the end of the month and into October 2014']}}]}]