Answer:
An open economy is one where trade of services and goods can easily take place. Here in this question, it is mentioned that the market for breakfast cereals is competitive, which means that there is not much room for profit making for the existing firms in the market since the products are also not much differentiated.
Hence, the conclusion drawn at the end is flawed because if the demand for cereals get increased and the price went up from $10 to $10.5, the market share for the higher price firm would decrease and people will move towards the firm which is still selling the cereals for $10, since it is a competitive market.
Hope this helps, Thank You.