3. A proton moving with mass m and speed v in a field-free region abruptly enters an essentially uniform magnetic field B as shown in the right figure (B ⊥ v). If the proton enters the magnetic field region at a 45° angle as shown, (A) the leaving angle θ of proton is 135°. (B) the leaving angle θ of proton is 45°. (C) the distance x exiting from the field is √2mv/eB. (D) the distance x exiting from the field is 2mv/eB. (E) the distance x exiting from the field is mv/2eB.