[单选题]
By what primary heat transfer mechani*tk *h ogqy:d4sm does one end of amig 3+y xo,y et-ns 4sz5uov2)n iron bar become hot when the other end is placed in a flam,m 54oe-ogx +svu2tn s3 yy)zie?
A. natural convection
B. conduction
C. radiation
D. forced convection
