一 What did Danny do yesterday?
一 He took part in the meeting.
对于我国自然资源的描述,正确的是( )
老年人易患骨质疏松症,是骨中缺少( )
已知抛物线y=ax2+bx+c的开口向下,顶点坐标为(2,-3),那么该抛物线有( )
若有氢气和氧气各一瓶(如图所示),其中B瓶盛放的气体是____ .