Question

freezers are placed at the top of refrigerators

Answer 1

Freezer is the source for the refrigerator's coldness. When thefreezer is placed on top, the cold air produced from it is denser than the warmer air in the bottom. So cold air being dense sinks down and the warm air is forced to rise up so when the warm air rises up it and gets cold in the freezer.
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Answer 2

freezer is the source for the refrigerator coldness. when the freezer is placed on top the cold air produced from it is denser than the warmer air in the bottom. so cold air being dense sinks down and the warm air is forced to rise up so when the warm air ruses up it and gets cold in the freezer
hope it will help you