Question: the number of elements in the fourth period is
[{'answer': '18', 'provenance': [{'wikipedia_id': '181554', 'title': 'Period 4 element', 'start_paragraph_id': 1, 'start_character': 449, 'end_paragraph_id': 1, 'end_character': 451, 'bleu_score': 1.0, 'section': 'Section::::Abstract.'}]}, {'provenance': [{'wikipedia_id': '181554', 'title': 'Period 4 element', 'start_paragraph_id': 1, 'start_character': 0, 'end_paragraph_id': 1, 'end_character': 660, 'bleu_score': 0.982060959997838, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 308240243708471236}, 'section': 'Section::::Abstract.'}]}]