如图,线段AC=6cm,线段BC=15cm,点M是AC的中点,在CB上取一点N,使得CN:NB=1:2,求MN的长.
2017年是西藏和平解放66周年。历史上元朝负责管理西藏事务的机构是( )
A survey of the opinions of the students _______ that two hours of physical exercise a day _______ acceptable in our campus.