爆炸极限是表征可燃气体、( )和可燃粉尘危险性的主要示性数。A．雾气 B．点火源

一个正方形的周长是c米，它的边长是（    ） 米，它的面积是（    ）平方米。

I’m glad to write to you. It is near a month since you had left our hospital. I’m still worried about you. How are you get along these days? I think you may be much better now, but I’d better

have a good rest and take good care of yourselves since you are sixty-year-old man. I still remember those days which you helped me to my English. With your help, I make good progress. But to my regret, my English remains very poor through I have made good use of my time. Can you give me some advice on it?

女性，60岁，反复咳嗽、咳痰10余年，每年冬季发作3个月以上。查体：桶状胸，双肺叩诊过清音，呼气音延长。其诊断可能为（）

A.肺结核

B.肺脓肿

C.慢性支气管炎并肺气肿

D.支气管哮喘

E.支气管肺癌

某省保健食品生产企业于2011年取得进口A保健食品批准证书。 保健食品批准证书到期时间为（）

A.2013

B.2014

C.2015

D.2016

填料密封由哪几部分组成？密封是依靠什么来保持密封的严密性？