气温由赤道向两极地区逐渐　____ 　，低纬度地区气温____ ，高纬度地区气温____ ；同纬度地带，夏季陆地气温____ ，海洋气温　____ ，冬季则相反．

依次填入下面句子中横线上的词语恰当的一项是 （    ）。

①仅仅________风沙袭击，固定沙丘阵地，还只是采取守势，自然是不够的。

②如何________核污染是核能利用中的一个大问题。

③增强自身免疫能力对________传染病至关重要。

 为了安全，汽车行驶时驾驶员必须系好安全带．当系好安全带时，相当于闭合开关，指示灯不亮；未系好安全带时，相当于断开开关，指示灯发光．符合上述要求的学电路图是（　　）

一个四位数的最高位是(    )位。

根据短文理解，选择正确答案。

MARS— The Red Planet

    “MARS—The Red Planet!” is one of our Early Reader Space books. It has surprising facts and photos explaining all about Mars. Aiming mainly at kids aged 7—10, it's an interesting way for readers to find out more about the red planet.

The Weather

    “The Weather” is one of our Early Earth books for boys and girls aged 13—15. This is a fun and fantastic way for readers to find out more about the weather on our earth. It offers information in a fun way—mixing facts, photos, and even has a web page with video clips explaining more about the information in the book.

Horses

    Kids Book of Fun Facts & Amazing Pictures on Animals in Nature — A Perfect Horse Book for Kids Aged 5—8.

    The book written by Kate describes the most noble animal on the planet for early readers. The book is packed with beautiful and clear pictures that help your child to understand the (fun) facts about this animal. It also includes anatomy(解剖) of horses.