At 19:30 on June 5, 2008, the Central Meteorological Observatory issued the weather forecast for Xi'an and Hong Kong in the next 24 hours: Xi'an, sunny, temperature 21℃

There is a lot of water in Hong Kong, and the specific heat capacity of water is larger. During the day, the same quality of water absorbs the same amount of heat as sand and gravel, but the temperature of the water rises less;

At night , releasing the same amount of heat, the temperature of the water drops less, making the temperature difference between day and night smaller in Hong Kong; while in the inland areas of Xi'an, where there is a lot of soil and sand, the specific heat capacity is small, and the temperature difference between day and night is large.

So choose B.