ANS:D - neither (a) and (b)

In a vertical tube evaporator, as the liquor level increases, both the velocity of circulation and the liquor-film coefficient typically increase. Here's why:

1. Velocity of Circulation: As the liquor level increases, the pressure head at the bottom of the evaporator also increases. This increased pressure head drives a higher flow rate or velocity of circulation of the liquid within the evaporator.
2. Liquor-film Coefficient: The liquor-film coefficient represents the rate at which heat is transferred from the heating medium (such as steam) to the liquid film flowing over the heating surface (the tube walls). With an increase in the liquor level, there is more liquid present in the evaporator, leading to a thicker liquid film over the heating surface. A thicker liquid film generally results in better contact between the liquid and the heating surface, which can increase the heat transfer rate and, thus, the liquor-film coefficient.