线间距是指两相邻线路之间的距离。

句型转换。

1. Helen's birthday is on November 26th.(对画线部分提问)

   ______ ______ Helen's birthday?

2. They have a School Day in June.(改为一般疑问句)

   ______ they ______ a School Day in June?

3. The Chinese speech contest is September 7th.(改为否定句)

     The Chinese speech contest______ ______ September 7th.

4. Her date of birth is March 16th.(改为同义句)

   ______ ______ is March 16th.

5. My sister is nine years old. (对画线部分提问)

    ______ ______ is your sister?

钾盐矿床的成矿机制。

合金的收缩分为几个阶段？各易产生何种缺陷？