How many bands are credited with the start of grunge music?
{'directed': True, 'multigraph': False, 'graph': {}, 'nodes': [{'type': 'QUESTIONS_ENTITY', 'name_': 'Q11365', 'id': 0, 'label': 'grunge'}, {'type': 'QUESTIONS_ENTITY', 'name_': 'Q215380', 'id': 1, 'label': 'musical group'}, {'type': 'ANSWER_CANDIDATE_ENTITY', 'name_': 'Q1127431', 'id': 2, 'label': 'Green River'}], 'links': [{'name_': 'P136', 'source': 2, 'target': 0, 'label': 'genre'}, {'name_': 'P31', 'source': 2, 'target': 1, 'label': 'instance of'}]}