Question: what does d stand for in arithmetic sequence
[{'answer': 'common difference', 'provenance': [{'wikipedia_id': '168389', 'title': 'Arithmetic progression', 'start_paragraph_id': 2, 'start_character': 89, 'end_paragraph_id': 2, 'end_character': 106, 'bleu_score': 1.0, 'section': 'Section::::Abstract.'}]}, {'provenance': [{'wikipedia_id': '168389', 'title': 'Arithmetic progression', 'start_paragraph_id': 6, 'start_character': 0, 'end_paragraph_id': 6, 'end_character': 108, 'bleu_score': 0.8852140475440831, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 3541078231510233735}, 'section': 'Section::::Abstract.'}]}]