$\sqrt{\left(3-a\right)\left(a+6\right)}$（﹣6≤a≤3）的最大值为（　　）

根据短文内容，回答下列问题。

    It's December. It's snowy and the weather is really cold in the north of China. You can see many children playing with snow（雪）. Some of them are making snowmen. Some are skating on the river, because it's so cold that the water in the river freezes(结冰). It's a white world. How beautiful! But in Australia now, the weather（天气）is hot. Many people go swimming in the sea. (大海) They are having a good time. Jim and his friends are playing soccer near the sea. The sunny weather is really cool to them. They want to swim after playing soccer.

    Why are they swimming in December? It is winter, isn't it? Yes, it is in China. But it isn't in Australia. That's because we, Chinese and Australians, have different seasons(季节) in a year.

“死去原知万事空，但悲不见九州同。王师北定中原日，家祭无忘告乃翁。”“现在，乡愁是一万浅浅的海峡，我在这头，大陆在那头。”这两首诗反映了作者的爱国情感，它们体现的民族精神是（  ）  

①家是她成长的摇篮，是她的港湾和第一所学校

②父母是他最亲近的人，也是她的第一任老师

③父母的行为迁就她，满足她所有的要求

④父母无微不至的关心她，爱护她，使她尽享家庭的温暖和亲情