[单选题]
By what primary heat1yun3p p6s *xv transfer mechanism does one end of an cbqd1: 9 jjyvdf4;11eewsm5q iron bar become hot when the other end is94fe1 5ew j qymcb:js1v;d1dq placed in a flame?
A. natural convection
B. conduction
C. radiation
D. forced convection
