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

(a) r4                           (b) r2 

(c) 2r                            (d) 4r

(d)   r=mvqBr1r2=v1v2×B2B1r1r2=12×12=14r2=4r1

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