A positively charged particle is at rest on the positive z-axis in reference frame S. Reference frame S′ is moving along the positive x-axis of S, reference frame S′′ is moving along the negative x-axis, and reference frame S′′′ is moving along the positive y-axis, all with speed v relative to S. The axes of all the reference frames are oriented the same way, and their origins coincide at instant t = 0, when observers in each reference frame make measurements.
In which reference frames do observers measure an electric field?
Check all that apply.

a) S b) S′ c) S′′ d) S′′′