In the design of a shell and tube heat exchanger, the corrosion allowance ____________________?

A. Need not be provided for non-pressure parts like tie rods, spacers, baffles, supports etc.
B. For carbon steel and cast iron pressure parts is 1.5 mm (except for tubes) and for severe
conditions it is 3 mm
C. For internal cover and tube sheet is provided on both the sides
D. All A., B. & C.