[单选题]
A cyclotron of radius 0op)xtab 9ge:) .25 m accelerates pro8 ej:czz0)x jz69-rin)reg; q anl7hdtons in a 1.7-T magnetic field. What is the kinetic energy of the protons when they leave the cyclothza8d ie z-lzx)c6nr0)n 7 e;9:jqjrgron?
A. $7.8 \; keV$
B. $8.7 \; keV$
C. $7.8 \; MeV$
D. $8.7 \; MeV$
