During the winter, a homeowner places a heavy cloth curtain across the stairwell leading upstairs from the main rooms. Which is the best explanation of the reason she does this?

A.
She wishes to place a radiating surface across the stairwell to help keep the area warm.
B.
She wishes to prevent the stairwell walls from drawing thermal energy away from downstairs by conduction.
C.
She wishes to keep hot air from downstairs from flowing up the stairwell by convection.
D.
She wishes to keep hot air from upstairs from flowing down the stairwell by convection.