The weather in Tancheng is 15 days.

There is more water in coastal areas, and the specific heat capacity of water is greater. During the day, compared with the quality of water and gravel, the absorbed heat is the same, and the temperature of water rises less.

At night, the same heat is released, and the water temperature drops less, making the temperature difference between day and night in coastal areas small. As can be seen from the table, the temperature difference in city B is small, so city B is the coastal city of Qingdao and city A is Linyi.

So choose D.