[单选题]
15.49: The kinetic energy associated with an electrp6u a ,caqlb)-hapx)2 o3z j3k o0cot/nn is twice its rest energy. At whot/3c 3nozjk0at speed is the electron traveling(in unit of m/s)?
A. $2.83\times\:10^8$
B. $2.67\times\:10^8$
C. $2.60\times\:10^8$
D. $2.25\times\:10^8$
E. $2.12\times\:10^8$
