The black people thus lay on the hard wooden boards, _________ stand up.

 After his journey from Beijing during the National Day, Tom returned school, __________.

Parents should be ____________ in the way they teach their children, so that the children know how to do.       

Smoking is one of the ________causes of cancer，killing millions of people each year. 

As to the subject, we have consulted the most _______ professor who gave us _______ explanation.