Question: when did o level grades change from numbers to letters
[{'answer': '1975', 'provenance': [{'wikipedia_id': '42062995', 'title': 'GCE Ordinary Level (United Kingdom)', 'start_paragraph_id': 6, 'start_character': 275, 'end_paragraph_id': 6, 'end_character': 279, 'bleu_score': 1.0, 'section': 'Section::::Grading.\n'}]}, {'provenance': [{'wikipedia_id': '42062995', 'title': 'GCE Ordinary Level (United Kingdom)', 'start_paragraph_id': 6, 'start_character': 0, 'end_paragraph_id': 6, 'end_character': 1839, 'bleu_score': 0.980098933374631, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 5102793221954449726}, 'section': 'Section::::Grading.\n'}]}]