An electron of mass m and charge q is travelling with a speed v along a circular path of radius r at right angles to a uniform of magnetic field B. If speed of the electron is doubled and the magnetic field is halved, then resulting path would have a radius of [Kerala PMT 2004; KCET 2000, 05]

• A.            \[\frac{r}{4}\]                      
• B.            \[\frac{r}{2}\]
• C.            \[2r\]                                       
• D.            \[4r\]