一个凸多边形共有20条对角线,它是几边形?是否存在有18条对角线的多边形?如果存在,它是几边形?如果不存在,说明得出结论的道理.
鼻腔内,鼻毛的作用是( )
Some experts believed, and _______ did we, kids who reach adolescence still need at least nine hours' sleeping.