Downtown Petropolis boasted over 100 large buildings 5 years ago. Since then, 60 of those buildings have been demolished. Since the number of large buildings in a downtown is an indicator of the economic health of that downtown, it is clear that downtown Petropolis is in a serious state of economic decline. Which one of the following is an assumption required by the argument? ['Significantly fewer than 60 new large buildings have been built in downtown Petropolis during the past 5 years.', 'There have never been significantly more than 100 large buildings in downtown Petropolis.', 'Most of the buildings demolished during the past 5 years were torn down because they were structurally unsound.', 'The large buildings demolished over the past 5 years have been replaced with small buildings built on the same sites.']