Compressed air is used in a heat-pump system to heat water, which is subsequently used to warm a house. The house demand is 95,000 Btu/h. Air enters the exchanger at 200°F and leaves at 120°F, and water enters and leaves the exchanger at 90°F and 125°F, respectively. Choose from the following alternative units the one that is most compact:

a. A counter flow surface with U = 30 Btu/h ft² °F and a surface-to-volume ratio of 130 ft²/ft³.

b. A cross flow configuration with the water unmixed and air mixed having U = 40 Btu/h ft² °F and a surface-to-volume ratio of 100 ft²/ft³.

c. A cross flow unit with both fluids unmixed with U = 50 Btu/h ft² °F and surface-to-volume ratio of 90 ft²/ft³.