[单选题]
By what primary heat xqd) 2cwm- ga.rc;ip f* uy2z/transfer mechanism d mm.euno.+6z, b3j/ v otce4eeoes one end of an iron bar become hot whe e/ o +ovmjbe 43t6zeme.u.c,nn the other end is placed in a flame?
A. natural convection
B. conduction
C. radiation
D. forced convection
