补全对话。

A:Hi, Jack. How     l    your vacation?

B:It was pretty good.

A:Where did you _  2     ?

B:I went to Beijing     3    my parents.

A:     4     did you go there?

B:We     5  _ there by train.

A:     6    did you do?

B:We visited Tian'an Men Square. There     7  _ many people there.

A:How were the people?

B:    8     were very friendly.

A:How was the     9     ?

B:    10     was sunny and cool.

耐张的零件，其机械强度安全系数不应小于（）。

A.2.0

B.2.5

C.3.0

D.4.0