Out of the Following Four Assumptions Used in the Derivation

Out of the following four assumptions used in the derivation of the equation for LMTD [LMTD = (?t1 - ?t2) /ln(?t1/?t2) ], which one is subject to the largest deviation in practice ?

A) Constant overall heat transfer co-efficient.
B) Constant rate of fluid flow
C) Constant specific heat
D) No partial phase change in the system