The opposite of the words given include the following below:
a. boiling - cooling
b. melting - freezing.
This is referred to as a physical process which involves the conversion of a solid to a liquid such as from ice to liquid water.
The opposite of this is freezing which involves solidification and can be described as the conversion of a liquid to solid.
Boiling on the other hand involves the application of heat to a substance while the opposite will be reduction of heat which is referred to as cooling thereby making them the correct choices.
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