15. An alpha particle (q = +2e, m = 6.644× kg) travels in a circular path of radius 9.00 cm in a uniform magnetic field with B = 1.20 T. Through what potential difference (volts) was it accelerated (from rest) to have the kinetic energy required for this path?
(A) 7.9×10³,
(B) 5.3×10⁴,
(C) 2.8×10⁵,
(D) 6.7×10⁶,
(E)3.1×10⁷.

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