An electron with kinetic energy 2.5keV moving along the positive direction of an x axis enters a region in which a uniform electric field of magnitude 10 kV/m is in the negative direction of the y axis. A uniform magnetic field B is to be set up to keeps the electron moving along the x axis, and the direction of B is to be chosen to minimize the required magnitude of B. In unit-vector notation, what B should be set up?