The weather in Xi 'an on June 5th.

Hong Kong has a lot of water, and its specific heat capacity is large. 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 temperature of water drops less, making the temperature difference between day and night in Hong Kong small; However, the inland area of Xi 'an has fertile soil, large sandstorm, small specific heat capacity and large temperature difference between day and night.

So choose B.