找工问题- 请指教

正在一些招工网站上发简历。有一些问题不清楚，请热心人指教。多谢！

1.Are you authorized to work for any employer in US?

Should I answer "yes" or "no"? I am a Canadian. In case I get an offer and visa, may I work for another employer in US?

2.Do you have security clearance?

don't understand this question.

3. Is there a big difference in monster website between a "public" and "private" resumes?