单项选择题
Which of the following conditions must exist for heat to flow from one object to another?（）
A.The two objects must be in physical contact
B.The two objects must be the same size
C.There must be an existing temperature differential
D.There must be an existing weight differential
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Which of the listed methods of heat transfer takes place when two substances of different temperatures are in physical contact with each other?（）
A.Radiation
B.Conduction
C.Convection
D.Each of the above
