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 at time t = 0.18 s if the magnitude of its angular momentum about the origin is
(a) 7.1 kg·m2/s,
(b) 7.1t2 kg·m2/s3,
(c) 7.1t1/2 kg·m2/s3/2, and
(d) 7.1/t2 kg·m2*s?