A real-estate development company owns a fully leased 40-storey office building. A tenant recently moved its offices out of two storeys of the building, leaving the space temporarily vacant. If the real-estate company considers moving its own offices into this 40-storey office building, what cost should it assign for the space? Is the cost of the vacant space zero, because the company paid for the building long ago, is it a cost that is sunk, or is there an incremental opportunity cost