Thank you for the opportunity to help you with your question!
When you heat air, you are adding energy to the molecules of the gas. Adding energy makes the particles move faster. When particles move faster, they hit the walls of the container more frequently and with greater force. So the answer would be the 3rd choice.
Please let me know if you need any clarification. I'm always happy to answer your questions.
Jan 15th, 2016
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