Halliday, Resnick & Walker — Rolling, Torque, and Angular Momentum: Problem 34_E

A particle is to move in an xy plane, clockwise around the origin as seen from the positive side of the z axis. In unit-vector notation, what torque acts on the particle if the magnitude of its angular momentum about the origin is (a) $4.0\text{ kg}\cdot\text{m}^2/\text{s}$, (b) $4.0t^2\text{ kg}\cdot\text{m}^2/\text{s}$, (c) $4.0\sqrt{t}\text{ kg}\cdot\text{m}^2/\text{s}$, and (d) $4.0/t^2\text{ kg}\cdot\text{m}^2/\text{s}$?