please list all moderate -ly priced restaurant -s in the centre of cambridge .
{'service': 'restaurant', 'act': 'request', 'slot_name': 'food', 'value': '?'} {'service': 'restaurant', 'act': 'inform', 'slot_name': 'choice', 'value': '21'}
there are 21 restaurant -s to choose from . would you like me to narrow this search down by choosing a type of food you would like to eat ?