能实现将转动变为直动的有哪几种机构？

阅读理解

       It’s a sunny and warm spring Sunday morning. There is no cloud in the sky. The sun is shining

brightly and the birds are singing happily. The Smith family are at home in the yard (院子). They are

doing their own things. Mr. Smith is planting flowers and trees. Mrs. Smith is drinking lemonade and

reading a book. Mary Smith is playing with the dog. Billy Smith is listening to the radio and dancing.

And Sam Smith is playing the guitar and singing some beautiful songs.The Smiths are very happy today.

It’s a beautiful day and they’re busy at home in the yard. What a nice day! What a happy family!

1. What is the weather like today?

                                                          .

2. What are Mr. Smith and Mrs Smith doing?

                                                          .

3. What is Billy doing?

                                                            .

4. What is Sam doing?

                                                            .

5. Is Mary playing with the dog?

                                                              .

风险事故发生的不确定性，是由内外部环境的复杂性和( )导致的。

A．多种风险因素共同作用
B．风险管理措施不力
C．风险事故的多样性
D．人们对未来事物变化的预测能力有限

（）利用用户之间的主动传播从而达到推广的目的。

A.电子邮件推广法

B.病毒性营销法

C.网络广告推广法

D.信息发布推广法

列举15种常见的野生鸟类，并指出其分类地位。

韶山3型电力机车每台转向架有（）轴箱独立悬挂装置。

A.2个

B.4个

C.6个

D.8个