As navigators, calendar makers, and other __________ of the night sky accumulated evidence to the contrary, ancient astronomers were forced to __________ that certain bodies might move in circles about points, which in turn moved in circles about the earth.

Solution

Calendar makers and someone watching  night sky are often reffered to as 'observers' and not scrutinizers. Scrutinizer means someone observing with very great care, which is not needed here. Out of 'concede' and 'agree' , concede matches perfectly with the context of statement as condede means - admint (to a wrongdoing) . Hence , option D.