Question: what are the names of the two rovers currently exploring mars
[{'answer': 'MER - A Spirit'}, {'answer': 'MER - B Opportunity'}, {'provenance': [{'wikipedia_id': '252908', 'title': 'Mars Exploration Rover', 'start_paragraph_id': 1, 'start_character': 0, 'end_paragraph_id': 1, 'end_character': 608, 'bleu_score': 0.8346372776231691, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 16789240781891653722}, 'section': 'Section::::Abstract.'}]}]