（   ）结构最稳定。

下列气态氢化物中最稳定的是（   ）

下图为某次会议签订的和约文本，该和约中最能体现这次会议实质的是（    ）

I met an old friend of my ____ in the street yesterday.(祖父母)

阅读短文，从方框中选择单词，根据实际情况用适当的形式，使短文意思完整、通顺。每空一词，每词只能用一次。

swim,  four,  snow,  after,  weather

    We know that there are ____ seasons（季节）in a year: spring, summer, autumn and winter. In spring, the ____ is warm. Everything grows fast. It is a good season for hiking (远足). Summer comes ____ spring. It is usually hot in summer. The children like to go ____. They are very happy in the cool water. It often rains and sometimes it rains heavily. In autumn, it is cool. The farmers are busy. It is often cold in winter. Sometimes it ____, and all the houses, trees and other things become white. The world is so beautiful in this season and people are very happy!