In the United States, building insulation is specified by the R-value (thermal resistance in h·ft²·°F/Btu units). A homeowner decides to save on the cost of heating the home by adding additional insulation in the attic. If the total R-value is increased from 15 to 25, the homeowner can expect the heat loss through the ceiling to be reduced by
(a) 25%

(b) 40%

(c) 50%

(d) 60%

(e) 75%