Consider a double-pipe heat exchanger with a tube diameter of 10 cm and negligible tube thickness. The total thermal resistance of the heat exchanger was calculated to be 0.025 K/W when it was first constructed. After some prolonged use, fouling occurs at both the inner and outer surfaces with the fouling factors 0.00045 m² · K/W and 0.00015 m² · K/W, respectively. The percentage decrease in the rate of heat transfer in this heat exchanger due to fouling is
(a) 2.3%

(b) 6.8%

(c) 7.1%

(d) 7.6%

(e) 8.5%