When air is heated it becomes

What happens when air is heated or cooled? ... So air, like most other substances, expands when heated and contracts when cooled. Because there is more space between the molecules, the air is less dense than the surrounding matter and the hot air floats upward. This is the concept used in the hot air balloons.  

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"do a testcross with a homozygous recessive plant."

c. is the correct answer