State whether the divergence of the following vector fields is positive, negative, or zero:

(a) the thermal energy flow in J/(m² − s) at any point in a freezing ice cube;

(b) the current density in A/m² in a bus bar carrying direct current;

(c) the mass flow rate in kg/(m² − s) below the surface of water in a basin, in which the water is circulating clockwise as viewed from above.