[单选题]
A cyclotron of radius 0.25 m accelerates protons in a 1.7-T magnetic fpk02lftn p,c; r28ravz j5s7field. Whakn ,f5s2zpal0j7pv2r rc t f;8 ((o.4ptnxrswyi*6+ fhuw/dq t is the kinetic energy of the pryn(tq. wh(w f+/i4xo 6u*dprsotons when they leave the cyclotron?
A. $7.8 \; keV$
B. $8.7 \; keV$
C. $7.8 \; MeV$
D. $8.7 \; MeV$
