Underline the topic sentence of each paragraph. 1 Glass-walled skyscrapers can be especially wasteful. 2 The heat loss (or gain) through a wall of half-inch plate glass is more than ten times that through a typical stone wall filled with insulation board. 3 To lessen the strain on heating and air-conditioning equipment, builders of skyscrapers have begun to use double-glazed panels of glass, and reflective glasses coated with silver or gold mirror films that reduce glare as well as heat gain. 4 However, mirror-walled skyscrapers raise the temperature of the surrounding air and affect neighboring buildings. for