A:is; would have
B:were; would have
C:was; would have had
D:were; would have had
鸟卵结构中,胚胎发育的主要营养来源是( )
空气的净化问题正日益受到人们的关注。下列各组中成分均属于空气污染物的一组是( )
如下图:已知平行四边形的面积是72平方厘米,阴影部分的面积____.