Cold water (c_p = 4.18 kJ/kg · K) enters a counter-flow heat exchanger at 10°C at a rate of 0.35 kg/s, where it is heated by hot air (c_p = 1.0 kJ/kg?K) that enters the heat exchanger at 50°C at a rate of 1.9 kg/s and leaves at 25°C. The effectiveness of this heat exchanger is
(a) 0.50

(b) 0.63

(c) 0.72

(d) 0.81

(e) 0.89