A stainless steel tube with inner and outer diameters of di = 22 mm and do = 27 mm, respectively, is used in a cross-flow heat exchanger. What is the area available for heat transfer per unit length of the tube?
a) π * (do² - di²)
b) π * do * di
c) π / 2 * (do + di)
d) 2 * π * (do + di)
