When does a moving charged particle not experience any force while moving through a uniform magnetic field?

The force experienced by a particle of charge $\mathrm{q}$  moving with velocity $\stackrel{\rightharpoonup }{\mathrm{v}}$ in a uniform magnetic field $\stackrel{\rightharpoonup }{\mathrm{B}}$ is given by $\stackrel{\rightharpoonup }{\mathrm{F}} = \mathrm{q}\stackrel{\rightharpoonup }{\mathrm{v} }\times \stackrel{\rightharpoonup }{\mathrm{B}}$ .

(a) Which pairs of vectors are always at right angles ?

The force experienced by a particle of charge $\mathrm{q}$ moving with velocity  $\stackrel{\rightharpoonup }{\mathrm{v}}$ in a uniform magnetic field $\stackrel{\rightharpoonup }{\mathrm{B}}$ is given by $\stackrel{\rightharpoonup }{\mathrm{F}} = \mathrm{q}\stackrel{\rightharpoonup }{\mathrm{v}} \times \stackrel{\rightharpoonup }{\mathrm{B}}$ .

(b) Which may have any angle between them ?