[CM.05]Timothy made the model shown below to represent the steps in the formation of a tornado. The diagram has four boxes. The first box on the top left has the symbol '?' in it, an arrow points from it, over to the left to a box with the text 'Severe thunderstorm'; this box has an arrow pointing down from it to a box with the text 'High winds push clouds'; and this box has an arrow pointing to the left to a box with the text 'Rotation within thunderstorm'.

© 2010 FLVS

Which of these best fits the empty cell in Timothy's model?

cool, dry air forms clouds
cool, dry air meets warm, moist air
water vapor condenses in clouds
water vapor evaporates from ocean