Question 1
Bubbles form on the walls of a beaker of water well before it reaches 100 °C. What are they?
The right answer is the one marked above.
Dissolved air is coming out of solution. Raising the temperature lowers the solubility of a gas, so the air that was dissolved in the cold water can no longer stay there.
This is the opposite of how most solids behave, and it is worth learning separately. Boiled water tastes flat precisely because almost no air is left dissolved in it.