Question: who is the presiding officer of the senate when the vice president is not there
[{'answer': 'President pro tempore', 'provenance': [{'wikipedia_id': '5626102', 'title': 'Presiding Officer of the United States Senate', 'start_paragraph_id': 3, 'start_character': 94, 'end_paragraph_id': 3, 'end_character': 115, 'bleu_score': 1.0, 'section': 'Section::::Abstract.'}]}, {'provenance': [{'wikipedia_id': '5626102', 'title': 'Presiding Officer of the United States Senate', 'start_paragraph_id': 3, 'start_character': 0, 'end_paragraph_id': 3, 'end_character': 795, 'bleu_score': 0.8285924603150041, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 18371710495610374900}, 'section': 'Section::::Abstract.'}]}]