被誉为“世界杂交水稻之父”的袁隆平爷爷，利用野生水稻经过多次杂交培育新品种，他采用的主要研究方法是(   )

A．观察法

B．调查法

C．分类法

D．实验法

根据下列句子的意思及所给出的汉语提示，写出空缺处的单词，固定短语或固定搭配的正确形式。

1. Please write an ________ (邀请) letter to________ (邀请) your pen friend.

2. Don't ________ (担心, 担忧) about your Chinese. We can help you.

3. It is ________ (必要的) for us to keep everything clean and tidy.

4. —Is China ________ from the USA? —Yes. There are many ________ between China and the USA. (不同).

5. We have a lot of friends ________ ________ ________(全世界)