Kitchen water at 10°C flows over a 10-cm-diameter pipe with a velocity of 1.1 m/s. Geothermal water enters the pipe at 90°C at a rate of 1.25 kg/s. For calculation purposes, the surface temperature of the pipe may be assumed to be 70°C. If the geothermal water is to leave the pipe at 50°C, the required length of the pipe is
(a) 1.1 m

(b) 1.8 m

(c) 2.9 m

(d) 4.3 m

(e) 7.6 m

(For both water streams, use k = 0.631 W/m . K, Pr = 4.32, v = 0.658 × 10^-6 m²/s, c_p = 4179 J/kg.K)