下列特点在动物进化上具有“划时代”意义的是（）。
A.中胚层的形成
B.同源器官
C.核膜的形成
D.脊索的形成
参考答案：A, D
某商品按原定价出售，每件利润是成本的20%，后来按原定价的90%出售，结果每天售出的件数比降件前增加了2.5倍，每天经营这种商品的总利润比降价前增加了______%．
用适当的介词填空。
1. There are two windows_____ the wall.
2. The broom is_____ the door. I can't see it.
3. There is a dog_____ the tree. It can't get down.
4. Where is the photo_____ your family?
5. There are some big apples_____ the tree.
